chemical metrology advanced school 17-21 november … · lgc setti ng standards in analytical...

LGC Setting standardsin analytical science

Chemical Metrology Advanced School

17-21 November 2003

Inorganic Analysis byIsotope Dilution Mass Spectrometry

Ruth Hearn

LGC, Teddington, UK


Overview

• Introduction to LGC

• Facilities of the Inorganic Team at LGC

• Isotope Dilution - basic principles

• Critical Stages of IDMS procedure

• Examples of Applications


LGC - The Laboratory of TheGovernment Chemist

1842: Founded as The Excise Laboratory• tobacco, beer and spirits; referee analyst

1911: Department of the Government Chemist

1959: Laboratory of the Government Chemist• expanded research role

1988: Government agency, move to Teddington• Office of Reference Materials and VAM

1996: LGC - a private company, part owned by RSC


Early chemical metrology at LGC

Sir Thomas Edward Thorpe, FRSPrincipal of the Laboratory, 1894-1909

Established first atomic weight table, 1903,with FW Clarke (USA) and K Seubert(Germany).

Honorary president of IUPAC AtomicWeights Committee until his death in 1924.


LGC’s Chemical Calibration Facility

• Inorganic analysis, organic analysis, separationscience

• The UK´s National Metrology Institute for chemistry

• Provision of reference values & reference materials

• Provision of advise & training on aspects oftraceability and quality to routine laboratories


The Chemical Calibration Team at LGC


The Inorganic Team• 10 permanent scientific members of staff

• Additional expertise within the Calibration Facility eg:– reference material production– uncertainty calculation

• Measurement technologies include– ICP-OES– collision cell ICP-MS– high sensitivity ICP-MS linked to LC/GC– magnetic sector ICP-MS– multi-collector magnetic sector ICP-MS

• Sample preparation facilities include high pressureclosed microwave, fusion apparatus and variousextraction techniques


Principle of Inorganic IDMS

• An isotopically enriched analogue (“spike”) of theanalyte element is added to a known mass of sample(= sample blend)

• The sample and spike materials area allowed toequilibrate (often by means of destructive digestion toa homogeneous aqueous solution)

• The ratio of two isotopes in this sample blend ismeasured using mass spectrometry

• Concentration of analyte in sample is calculatedbased on the spike concentration, the ratio ofisotopes and the weights of sample and spike


Single IDMS

• Reliant on accurate quantification of enrichedanalogue

• Measured ratio requires correction for– mass bias and– detector linearity (if ratio is not = 1)


Exact Matching Double IDMS

• A calibration standard is spiked with the sameisotopic analogue as the sample (= calibration blend)

• Sample blends and calibration blends are preparedsuch that :– the isotope ratios in the blends are the same– the intensities of the peaks in the blends are the

same



• This is an iterative process as the concentration ofthe analyte must be determined before exactmatching can occur

• Advantages of this method :– the effects of mass bias and linearity are

eliminated since both calibration and sampleblends are affected equally

– accurate quantification of the isotopic analogue isnot required


Critical Factors for Inorganic IDMS

• Sample & spike equilibration

• Freedom from spectral interferences

• Selection of analyte and spike isotopes

• Choice of spiking ratio

• Measurement of Isotopic Abundances

• Blank consideration

• Instrument optimisation

• Measurement sequencing


Critical Factors -Sample & Spike Equilibration

• It is essential that the analyte and spike isotopes areequilibrated to ensure identical behavior during theanalytical procedure

• Spike should be added to the sample at the earlieststage of the process

• In many cases e.g. solid samples, analyte and spikecan only be equilibrated after destructive digestion toa homogenous, aqueous solution.

• If analyte is volatile, the sample and spike must bekept in a closed system until equilibration isachieved


Critical Factors -Freedom from Spectral Interferences

• Both analyte and spike isotopes should be free ofspectral interferences. These are most likely to occurin the sample blend due to the sample matrix

• This should be checked by comparing unspikedratios in a calibration standard and a preparedsample

• If a significant deviation is observed, then theinterference should be eliminated by instrumentalmeans (eg high mass resolution or collisional gas, bymathematical correction for the interference or byselection of a different isotope pair)


Critical Factors -Selection of Isotopes

• Availability of enriched analogues and freedom frominterference will usually dominate decisions onisotope selection

• For optimum isotope ratio precision, the ratio insample and calibration blends should be = 1

• Therefore, it is common to select a major isotope forthe analyte and a minor isotope for the spike

• The two isotopes should be as close in mass aspossible to optimise precision


Critical Factors -Choice of Spiking Ratio

• It may not always be practical to use a blend ratio = 1

• For example, Ag has a natural ratio of 1.08 andtherefore, spiking to a ratio of 1.0 would not bepractical

• The optimum ratio can be calculated using the errorpropagation factor below:

• In general, it is advised to use a ratio between 0.25and 4

Abundance SampleMain Abundance SampleMinor

Abundance SpikeMinor Abundance SpikeMain

×= EPF opt


Critical Factors -Blank Consideration

• A full reagent blank, subjected to all the samplepreparation steps, should be used to evaluatepossible blank contributions

• Ideally, the calibration blend should also be putthrough the same sample preparation procedures sothat sample and calibration blends are subject to thesame sources of blank contamination

• If significant levels of analyte are found in the blank,then separate quantification of the blank by IDMS isrecommended


Critical Factors -Instrument Optimisation

• In most cases, the most dominant source ofuncertainty for inorganic IDMS is the precision of theisotope ratio measurements.

• Therefore, the instrument must be carefully tuned togive the best possible measurement stability

• The analyte concentration in the measurementsolution should be sufficient to give a signalsignificantly higher than the instrument background

• If simultaneous detection of the two isotopes (multi-detectors) is not available, the scanning parametersshould be selected such that isotopes are scannedrapidly with many repetitions


Critical Factors -Measurement Sequencing

• Mass bias drifts may occur between measurement ofa sample blend and a calibration blend

• Therefore, each measurement of sample blendshould be bracketed (before & after) with a calibrationblend

• The calibration ratio used in the calculation should bethe average of the leading and trailing calibrationblend for each sample

• This drift should be monitored during measurementand should not be allow to exceed a few percent(depending on the final uncertainty requirement)


If all the critical factors have beenaddressed, does that mean we have

the right answer?

• Not necessarily - we must perform as manyconfirmation and validation experiments aspossible....


Confirmation & Validation Steps

• Use at least two independent calibration standardspreferably from two sources

• Measure as many suitable reference materials aspossible

• Check robustness of method by altering theprocedure slightly

• Have a second analysts perform the same procedure

• Use a different sample preparation technique

• Use a different measurement technique

• Use a different isotope ratio


Applications of LGC’sInorganic IDMS Methods

• Sulfur in fuel

• Organo-tin compounds in sediment

• CCQM studies



Part 1: Sulfur in Fuel

• Legislative limits are set to reduce levels of sulfur infuels to 50 µg/g

• However, there is increasing demand for methods foranalysing sulfur in fuel as low as 10 µg/g

• LGC has developed an IDMS method at these levels

• The method has been used to provide referencevalues for a European round robin project

• And for the certification of reference materials


High Resolution Inductively CoupledPlasma Mass Spectrometry


CEN WG27 Results for PetrolIDMS result and uncertainty indicated by horizontal lines

15

1617

1819

20

2122

2324

25W

DXR

F H

P

WD

XRF

LP

WD

XRF

HP

WD

XRF

LP

UVF

EDXR

F N

P

EDXR

F P

Mic

roco

ulom

etry

Con

cent

ratio

n S

ug/g


CEN WG27 Results for DieselIDMS result and uncertainty indicated by horizontal lines

24

26

28

30

32

34

36

38

40W

DX

RF

HP

WD

XR

F LP

WD

XR

F H

P

WD

XR

F LP

UVF

ED

XR

F N

P

ED

XR

F P

Mic

roco

ulom

etry

Con

cent

ratio

n of

S in

ug


LGC S in Fuel Reference Materials

• 6 diesel reference materials obtained for certification:- 450, 100, 50, 30, 10 and 1 µg/g

• 1 gasoline reference materials currently beingorganised- probably at 20 or 10 µg/g

• Certification requires homogeneity testing, stabilitytesting and assignment of concentration value - alldetermined by isotope dilution

• Certification of the 50 µg/g material is complete.Expanded uncertainty of 2.5%

• Certification of the 10 µg/g material is underway.Expected completion Jan 2004



Part 2: Tributyl tin in sediment• Organotin compounds are used as:

– Fungicide, bactericide and anti-fouling agent in paint

– Preservatives in wood, textile, paper, leather and glass

– Industrial applications: catalyst, PVC stabilisers

• Production has paralleled PVC production:– 70 - 90% of OT produced as PVC stabilisers, remainder

as biocides

• Environmental concerns and human healthconcerns


Tributyl tin in sediment

• Use an isotopically enriched tributyl117Sn for IDMS

• Sample preparation must be such that there is nobreakdown of species

• Extraction of species in sample must be complete inorder to achieve full equilibration of sample and spikespecies

• Chromatographic separation is required to isolatespecies of interest before mass spec detection of tinisotopes


HPLC-ICP-MS for Organometallic Studies


Extraction procedureSample

Accelerated Solvent Extraction (ASE)0.5M sodium acetate/ 1.0M acetic acid in methanol

(after Arnold et.al.,1998)Extraction- 3 min Pressure- 1500 psi Temperature- 100 ºC

Aliquot diluted with H2O

For HPLC-ICP-MS analysis

IDMS:

Spiking with117Sn enriched

compound

20µL injection

For GC-ICP-MS analysis

Derivatisation with NaBEt4

1µL injection

Liquid extraction in hexane


Sediment TBT results

Sample LC (ng/g as Sn)

Uncertainty (ng/g)

GC (ng/g as Sn)

Uncertainty (ng/g)

1 827 19 853 122 805 38 846 133 845 9 838 8Mean 826 22 846 11Std Dev 20 15 8 3RSD 2.4 67 0.9 24

• Sample extracted in triplicate

• 4 injections of each extract

• Each injection bracketed by mass-bias injections


0.6

0.7

0.8

0.9

1

1.1

BAMCro

mpton

IRMM

LGC

LNELim

nolNARL

NIMJ

NRCC

PauOvie

do

Nor

mal

ized

TB

TMean: 0.875 ± 0.094

Median: 0.858

TBT in CCQM P-18 Sediment



Part 3: A selection of CCQM studies

• LGC has participated in 15 CCQM key comparisonsand pilot studies for inorganic analytes

• Submitted results for 15 inorganic analytes (includingthree organometallics)

• These analytes have been analysed in 8 matricesranging from mono-elemental solutions to foods tosteel


LGC

2.05

2.1

2.15

2.2

2.25

2.3

2.35

2.4

2.45

CCQM-K14 : Calcium in serumAll results

Results of all participants (9 laboratories)(Pink l ine= averag e of the 9 results)

(Blu e dotted lin e = 1 Standard deviation)


LGC

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

c

10

-5

CCQM-P39: Se in tuna fish8.098 ± 0.745 ·10-5

Invited expert laboratories


LGC

0.11

0.12

0.13

0.14

0.15

Pb a

mou

nt c

onte

nt (10

-9 m

ol/g

)CCQM-P12: Pb in Wine


Conclusions & Acknowledgements

• LGC’s inorganic IDMS methods have been used toprovide reference values and reference materials toUK industry

• The methods provide full traceability to SI units andhave been proved comparable in severalinternational intercomparisons

• Acknowledge the funding from the UK Department ofTrade & Industry VAM Programme and to all mycolleagues at LGC

chemical metrology advanced school 17-21 november … · lgc setti ng standards in analytical...

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