Staying FocussedStaying Focussed

An introduction to stable An introduction to stable isotope mass spectroscopyisotope mass spectroscopy

Sample preparationSample preparationChemically convert sample material (ie rocks water biological materials) Chemically convert sample material (ie rocks water biological materials)

into gasinto gas

QuantitativeQuantitative

Measurement of isotope ratiosMeasurement of isotope ratiosMass spectroscopyMass spectroscopy

Laser cavity molecular spectroscopyLaser cavity molecular spectroscopy

Normalization of resultsNormalization of resultsLaboratory referencesLaboratory references

International standardsInternational standards

Stable Isotope analysis

Stable Isotope mass specs are Stable Isotope mass specs are gas-sourcegas-source

DH DH HH22

1818OO16O O COCO2 CO O CO O2

13C12C COCO2 CO CO1515NN1414N N NN22

3434SS3232S S SOSO22 SO SF SO SF66

3737ClCl3535ClCl CHCH33ClCl

CombustCombust(C(C66HH1010OO55))nn + O + O22 COCO22 + H + H2200

(C(C66HH1010OO55))nn + C + C CO + HCO + H22

Reduce

H20 + Zn H2 + ZnO

Equilibrate

C16O2 + 2H218O C16O18O + 2H2

16O18O

React

SiO2 + BrF5 O2 + SiF2

O2 + C CO2

PurificationPurificationVacuum linesVacuum lines

Cryogenic (LN2) traps for Cryogenic (LN2) traps for separation of gassesseparation of gasses

Reaction vessels for chemical Reaction vessels for chemical reactions in vacuumreactions in vacuum

Usually used in conjunction Usually used in conjunction with ldquooff-linerdquo isotope analysiswith ldquooff-linerdquo isotope analysis

Necessary for some analyses Necessary for some analyses ie silicate analysesie silicate analyses

Gas ChromatographyGas Chromatography

Uses a GC column to separate gassesUses a GC column to separate gasses

HeHe

TCD detectorsTCD detectors

usually in a singe instrument as a usually in a singe instrument as a preparatory inlet to a mass preparatory inlet to a mass spectrometerspectrometer

CombustionReductionCombustionReduction

AutomationAutomation

ldquoldquoOn-LinerdquoOn-Linerdquo

MassSpec

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Sample preparationSample preparationChemically convert sample material (ie rocks water biological materials) Chemically convert sample material (ie rocks water biological materials)

into gasinto gas

QuantitativeQuantitative

Measurement of isotope ratiosMeasurement of isotope ratiosMass spectroscopyMass spectroscopy

Laser cavity molecular spectroscopyLaser cavity molecular spectroscopy

Normalization of resultsNormalization of resultsLaboratory referencesLaboratory references

International standardsInternational standards

Stable Isotope analysis

Stable Isotope mass specs are Stable Isotope mass specs are gas-sourcegas-source

DH DH HH22

1818OO16O O COCO2 CO O CO O2

13C12C COCO2 CO CO1515NN1414N N NN22

3434SS3232S S SOSO22 SO SF SO SF66

3737ClCl3535ClCl CHCH33ClCl

CombustCombust(C(C66HH1010OO55))nn + O + O22 COCO22 + H + H2200

(C(C66HH1010OO55))nn + C + C CO + HCO + H22

Reduce

H20 + Zn H2 + ZnO

Equilibrate

C16O2 + 2H218O C16O18O + 2H2

16O18O

React

SiO2 + BrF5 O2 + SiF2

O2 + C CO2

PurificationPurificationVacuum linesVacuum lines

Cryogenic (LN2) traps for Cryogenic (LN2) traps for separation of gassesseparation of gasses

Reaction vessels for chemical Reaction vessels for chemical reactions in vacuumreactions in vacuum

Usually used in conjunction Usually used in conjunction with ldquooff-linerdquo isotope analysiswith ldquooff-linerdquo isotope analysis

Necessary for some analyses Necessary for some analyses ie silicate analysesie silicate analyses

Gas ChromatographyGas Chromatography

Uses a GC column to separate gassesUses a GC column to separate gasses

HeHe

TCD detectorsTCD detectors

usually in a singe instrument as a usually in a singe instrument as a preparatory inlet to a mass preparatory inlet to a mass spectrometerspectrometer

CombustionReductionCombustionReduction

AutomationAutomation

ldquoldquoOn-LinerdquoOn-Linerdquo

MassSpec

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Stable Isotope mass specs are Stable Isotope mass specs are gas-sourcegas-source

DH DH HH22

1818OO16O O COCO2 CO O CO O2

13C12C COCO2 CO CO1515NN1414N N NN22

3434SS3232S S SOSO22 SO SF SO SF66

3737ClCl3535ClCl CHCH33ClCl

CombustCombust(C(C66HH1010OO55))nn + O + O22 COCO22 + H + H2200

(C(C66HH1010OO55))nn + C + C CO + HCO + H22

Reduce

H20 + Zn H2 + ZnO

Equilibrate

C16O2 + 2H218O C16O18O + 2H2

16O18O

React

SiO2 + BrF5 O2 + SiF2

O2 + C CO2

PurificationPurificationVacuum linesVacuum lines

Cryogenic (LN2) traps for Cryogenic (LN2) traps for separation of gassesseparation of gasses

Reaction vessels for chemical Reaction vessels for chemical reactions in vacuumreactions in vacuum

Usually used in conjunction Usually used in conjunction with ldquooff-linerdquo isotope analysiswith ldquooff-linerdquo isotope analysis

Necessary for some analyses Necessary for some analyses ie silicate analysesie silicate analyses

Gas ChromatographyGas Chromatography

Uses a GC column to separate gassesUses a GC column to separate gasses

HeHe

TCD detectorsTCD detectors

usually in a singe instrument as a usually in a singe instrument as a preparatory inlet to a mass preparatory inlet to a mass spectrometerspectrometer

CombustionReductionCombustionReduction

AutomationAutomation

ldquoldquoOn-LinerdquoOn-Linerdquo

MassSpec

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

CombustCombust(C(C66HH1010OO55))nn + O + O22 COCO22 + H + H2200

(C(C66HH1010OO55))nn + C + C CO + HCO + H22

Reduce

H20 + Zn H2 + ZnO

Equilibrate

C16O2 + 2H218O C16O18O + 2H2

16O18O

React

SiO2 + BrF5 O2 + SiF2

O2 + C CO2

PurificationPurificationVacuum linesVacuum lines

Cryogenic (LN2) traps for Cryogenic (LN2) traps for separation of gassesseparation of gasses

Reaction vessels for chemical Reaction vessels for chemical reactions in vacuumreactions in vacuum

Usually used in conjunction Usually used in conjunction with ldquooff-linerdquo isotope analysiswith ldquooff-linerdquo isotope analysis

Necessary for some analyses Necessary for some analyses ie silicate analysesie silicate analyses

Gas ChromatographyGas Chromatography

Uses a GC column to separate gassesUses a GC column to separate gasses

HeHe

TCD detectorsTCD detectors

usually in a singe instrument as a usually in a singe instrument as a preparatory inlet to a mass preparatory inlet to a mass spectrometerspectrometer

CombustionReductionCombustionReduction

AutomationAutomation

ldquoldquoOn-LinerdquoOn-Linerdquo

MassSpec

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

PurificationPurificationVacuum linesVacuum lines

Cryogenic (LN2) traps for Cryogenic (LN2) traps for separation of gassesseparation of gasses

Reaction vessels for chemical Reaction vessels for chemical reactions in vacuumreactions in vacuum

Usually used in conjunction Usually used in conjunction with ldquooff-linerdquo isotope analysiswith ldquooff-linerdquo isotope analysis

Necessary for some analyses Necessary for some analyses ie silicate analysesie silicate analyses

Gas ChromatographyGas Chromatography

Uses a GC column to separate gassesUses a GC column to separate gasses

HeHe

TCD detectorsTCD detectors

usually in a singe instrument as a usually in a singe instrument as a preparatory inlet to a mass preparatory inlet to a mass spectrometerspectrometer

CombustionReductionCombustionReduction

AutomationAutomation

ldquoldquoOn-LinerdquoOn-Linerdquo

MassSpec

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Gas ChromatographyGas Chromatography

Uses a GC column to separate gassesUses a GC column to separate gasses

HeHe

TCD detectorsTCD detectors

usually in a singe instrument as a usually in a singe instrument as a preparatory inlet to a mass preparatory inlet to a mass spectrometerspectrometer

CombustionReductionCombustionReduction

AutomationAutomation

ldquoldquoOn-LinerdquoOn-Linerdquo

MassSpec

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Mass spectrometersMass spectrometers

JJ Thomspon 1910 ndash parabola JJ Thomspon 1910 ndash parabola spectrographspectrographbull Discovered first stable isotopes (Ne mass 20-22)Discovered first stable isotopes (Ne mass 20-22)bull Discovered the electronDiscovered the electronbull Awarded nobel prize 1906Awarded nobel prize 1906

FWAston ndash mass spectrographFWAston ndash mass spectrographbull Discovered 21NeDiscovered 21Nebull 212 out of 287 naturally occurring isotopes212 out of 287 naturally occurring isotopesbull Mass defect ndash binding energiesMass defect ndash binding energiesbull Awarded nobel prize 1922Awarded nobel prize 1922

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

A NierA Nierbull First stable isotope abundance instrumentFirst stable isotope abundance instrumentbull Electron-impact sourceElectron-impact sourcebull Dual detectors Dual detectors bull Magnetic sectorMagnetic sectorbull Electronic rather than photographic ion countingElectronic rather than photographic ion counting

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

A lot has changed a lot has A lot has changed a lot has stayed the samehelliphellipstayed the samehelliphellip

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Pumping systemDiffusion or turbo pumps

Detectors-Faraday cups-electronic ion counting

Analyser = Magnetic sector

Source of ionsM+

Inlet System DI or CF

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Ion SourceIon SourceElectron Impact source M + eElectron Impact source M + e-- MM++ + 2e + 2e--

Electron energies ca 100 eVElectron energies ca 100 eVElectron emission 1mA or 6x10Electron emission 1mA or 6x101515 e-s e-sEfficiency = 1 in 2400 molecules ionizedEfficiency = 1 in 2400 molecules ionized

ProblemsProblems-Linearity -Linearity current ne const current ne const

measured ratiosmeasured ratios

-memory-memory

-stability-stability

-chemical inertness of hot filiament-chemical inertness of hot filiament

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Ionization efficiency - sensitivityIonization efficiency - sensitivity Electrons about 70 eV ndash de Electrons about 70 eV ndash de

Broglie Broglie is about equal to is about equal to molecule bond lengthsmolecule bond lengths

About 1 in 1000 impacts give About 1 in 1000 impacts give ionizationionization

Emission is about 1 mA Emission is about 1 mA Cross section is low ndash 10Cross section is low ndash 10-7-7 mm mm22

About 1nA ion current from About 1nA ion current from 1mA emission 1mA emission

Increased source pressure- Increased source pressure- closed ion boxclosed ion box

Potential across ion box ndash too Potential across ion box ndash too high ndash variable ion energies ndash high ndash variable ion energies ndash too low ion-molecule too low ion-molecule interactionsinteractions

fragmentationfragmentation

Ca 70V

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Ion opticsIon opticsbull About 50 efficientAbout 50 efficientbull Burn marksBurn marksbull ExtractionExtractionbull Half plate focussingHalf plate focussingbull Fine-tuning ndash Fine-tuning ndash bull Generally Generally

empirically tunedempirically tuned

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

11 Deviations from linear behavior ndash ldquodiscriminationrdquoDeviations from linear behavior ndash ldquodiscriminationrdquo ion-molecule interactions forming isobaric ion-molecule interactions forming isobaric

interferences ndash ie H3+interferences ndash ie H3+ collimating magnetic field can lead to non-linear collimating magnetic field can lead to non-linear

responseresponse changes in number of ions ndash affects space charge of changes in number of ions ndash affects space charge of

ion-source ndash therefore extraction conditionsion-source ndash therefore extraction conditions careful source designcareful source design more important in CF instrumentsmore important in CF instruments

Problem areas of source designProblem areas of source design

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Problem areas of source designProblem areas of source design

2 Gas exchange2 Gas exchangebull Minimize gas exchange in ion sourceMinimize gas exchange in ion sourcebull Pumping efficiency of source regionPumping efficiency of source regionbull Avoidance of dead volumesAvoidance of dead volumesbull Chemical inertness of source materialsChemical inertness of source materialsbull Filament ndash chemical inertness and Filament ndash chemical inertness and

conditioningconditioning

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Problem areas in ion sourcesProblem areas in ion sources

3 High stability over time3 High stability over time electrostatic potentials need to be electrostatic potentials need to be

stable to 200 ppmstable to 200 ppm insulating surface layers lead to insulating surface layers lead to

charging ndash source cleaningcharging ndash source cleaning

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

AnalyserAnalyser Magnetic SectorMagnetic Sector

bull U=HT B=Mag Field z = charge e= charge m=massU=HT B=Mag Field z = charge e= charge m=mass

mz 44 (CO2) at 5KV = 135 cm B=05Tmz 44 (CO2) at 5KV = 135 cm B=05T PermanentelectromagnetsPermanentelectromagnets Magnetic field more uniform with electromagnetsMagnetic field more uniform with electromagnets Need two magnets for low mass Need two magnets for low mass Image broadening by inhomogeneous ion energyImage broadening by inhomogeneous ion energy Large-radius ndash high energy (10kV) less affectedLarge-radius ndash high energy (10kV) less affected EU less in large radius instrumentsEU less in large radius instruments Early instruments x-only focussingEarly instruments x-only focussing All modern instruments X-Y (cross)focussingAll modern instruments X-Y (cross)focussing

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Cross focussingCross focussing

Stigmatic focussingStigmatic focussing Ions enter and exit magnet at an Ions enter and exit magnet at an

offset angle rather than 90degoffset angle rather than 90deg Fringing fields at the magnet Fringing fields at the magnet

pole gap result in y-direction pole gap result in y-direction focussingfocussing

Mat 250 ndash 1977Mat 250 ndash 1977

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Permanent vs ElectromagnetsPermanent vs Electromagnets From a theoretical veiwpoint both are identicalFrom a theoretical veiwpoint both are identical Limited mass selection with permanent magnets ndash Limited mass selection with permanent magnets ndash

5KV for N2 (typically designed at high end of HT 5KV for N2 (typically designed at high end of HT range)range)

32 kV for CO232 kV for CO2

22 kV for SO2 (v low HT- lower resolution)22 kV for SO2 (v low HT- lower resolution)

cannot scan lower than mass 28 without magnet changecannot scan lower than mass 28 without magnet change

HD separate magnetHD separate magnet

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

DetectorsDetectorsFaraday cupsFaraday cups

Mechanically simpleMechanically simple

Named after Micheal Farrady who first theorized ions about 1830Named after Micheal Farrady who first theorized ions about 1830

Circuit where charged ions are the charge carriers in vacuum

Cup gains charge that can be measured as current when discharged

Nt = IeNt= of ionssec I= current e= elementary charge(16x10-

19 C)

1nA = 6x109 ionssec

Error sources-Secondary electrons-backscattering

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

ElectrometersElectrometers

Measure charge or currrent (chargesec)Measure charge or currrent (chargesec) Solid state ndash transistorsSolid state ndash transistors Ohms law E=IR Ohms law E=IR 1V = 101V = 10-9-9A x 10A x 109 9 ohmsohms Different resistors ndash different currents - similar Different resistors ndash different currents - similar

voltages voltages Measured by ADCMeasured by ADC High amplifications ndash shieldedHigh amplifications ndash shielded

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Masses not evenly spaced ndash so cant get a collector array for more than element

-compromise ndash triple array or moving collectors

r m

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Flat-topped peaks ndash defining slit widthFlat-topped peaks ndash defining slit width

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

InletsInlets

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

McKinney (1950) ndash introduced change-over valve thereby eliminating most instrumental effects allowed measurement of O2 and CO2 to 01 per mil-Measured d13C to precisions of about 01 per mil-Has essentially remained unchanged in 50 years

-smallest sample limited by requirement to maintain viscous-flow conditions-Practical limit about 15-20 mbar-Cold fingers for small volumes-Smallest sample size about 02 mol- With a few exceptions most sample preparations are ldquooff-linerdquo

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Continuous Flow or IR monitoring inlet

bullGC techniques coupled to MSbullNo change over or dual inletbullViscous flow in GC streambullSmaller sample sizesbullCompletely taken over most modern analysesbullWell suited for automated analyses

Things to be aware ofo Linearity effectso small measurement timeso absolute sensitivityo isotope chromatographyo statistical limits on precisiono large He background (HD)o Background corrections

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Isotope ChromatographyIsotope Chromatography Transport of gas through GC Transport of gas through GC

not only separates chemical not only separates chemical species but also isotopic species but also isotopic speciesspecies

Cannot measure instantaneous Cannot measure instantaneous isotope ratios but must isotope ratios but must integrate entire peaks to ldquocount integrate entire peaks to ldquocount ionsrdquoionsrdquo

Makes correct background Makes correct background subtraction and peak subtraction and peak integration algorithms essentialintegration algorithms essential

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

HD measurement in HeHD measurement in He Large mass 4 (He+) tails into mz Large mass 4 (He+) tails into mz

33 Generally reduced by modern Generally reduced by modern

instrument designinstrument design Differential pumpingDifferential pumping Increased abundance sensitivity Increased abundance sensitivity

by increased dispersionby increased dispersion Energy filters to homogenize Energy filters to homogenize

minimize minimize UU

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Statistical Limits to precisionStatistical Limits to precision IRMS is basically measuring ion IRMS is basically measuring ion

currentscurrents Ion currents have a standard deviation Ion currents have a standard deviation

as a result of ldquoshot noiserdquoas a result of ldquoshot noiserdquo For low ion currents - poisson For low ion currents - poisson

distribuiondistribuion - = 1radicN- = 1radicN

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Implications for CF-IRMS

-Typically small total ion numbers are measured

For example typical 10nA CO2 peak ndash about 3x1011 ions for mass 44-about 3x109 ions for mass 45

= 1radic3x109 = 2x10-5 or about 002 per mil

Reference peak has similar precisions so that minimum statistical limit of is about 005 permil or so Minimum

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Mass Resolution

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Abundance SensitivityAbundance Sensitivity

Basically how much does one mass Basically how much does one mass peak overlap the otherpeak overlap the other

About 10-5 on modern instruments for mz=45=0001 per mil

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Instrument Corrections

Corrections to measured d values based on instrument properties

Not as important on newer instruments as manufacturing and materials have improved

Should be monitored to evaluate instrument performance

1 Leak correction (zero enrichment) - corrects for differences in the two viscous leaks- crimp is adjusted so that enrichment is zero

2 Abundance sensitivity- effect of one mass on the adjacent mass- principally controlled by instrument design- is different for each mass- dependent on inlet pressure

3 Valve mixing-mixing of reference and sample gasses in the changeover valve by cross seat leakage- new changeovers minimize this

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Computation of values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Optical MethodsOptical MethodsTwo Competing technologies 1 Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)

Picarro Inc2 Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research

HDO

H2O

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

bullAbsorption spectrometry is a direct measure of concentration

bullVery selective - C2H2 absorbs light between 1510 - 1545 nm

bullFast ndash laser can be reproducibly swept at gt 100 Hz

bullFor a 1 meter sample containing 100 torr of 1 ppm acetylene ΔII0 ~ 10-5

bullIncrease pathlength by (1-R)-1 ~ 10000 times giving several kilometers of effective pathbullSingle-pass ΔII0 ~ 10-5 1048890 Multipass ΔII0 ~ 10-1 (a considerable absorption)

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Pros - consPros - cons

Do one thing really well ndash eg Do one thing really well ndash eg H2OH2O

No compressed gassesNo compressed gasses No moving partsNo moving parts CheapCheap Simple mechanicallySimple mechanically

ProsCons

Do one thingDo one thing Difficult to calibrateDifficult to calibrate Like IRMS ndash instrumental Like IRMS ndash instrumental

effectseffects

Wikipedia ndash A modern stable isotope ratio mass spectrometer

Wikipedia ndash A modern stable isotope ratio mass spectrometer