measurement of gas concentrations by mass spectrometers günter peter, n. müller, w. neff, bled...

Measurement of Gas Concentrations by Mass Spectrometers

Günter Peter, N. Müller, W. Neff, Bled April 2012

Quadrupole Mass Spectrometers are

gas-specific instruments, the fragmentation pattern may lead to spectral overlap

several sources of instabilities

how to operate the instrument if the SEM or C-SEM is the major source of drift?

spectral overlap of gaseous components lead to

MCD-Measurements

how to calibrate

some (two) examples

check whether re-calibration is required


Günter Peter, Bled April 2012

Functional units of every Quadrupole Mass-Spectrometer:

An ion-current, amplified ion-current or a count rate is the original out-put of these instruments.

Faraday

U+Vcost

Electron multiplier

Ion sourcesupply

RFgenerator

HVsupply

Electrometerpreamplifier

Quadrupole controller

Ion sourceMass filter



Ion-Source

The amount of ions generated in the ion source depends on the partial pressure ofthe individual species and on the energy of the electrons used for electron impact ionization.

As with every ionization gauge, the signal generated is gas-specific.



Ionization Probability compared to nitrogen (electron energy : 70 eV)

The same current for He and Ar makes a difference in pressure of a factor of 8.



Stability of Quadrupole Mass Spectrometers: (see*)

The cracking pattern and the resolution versus mass range may change.

The mass-scale may shift.

The amplification of the SEM or C-SEM may drift.

* 8th European Vacuum Congress EVC-8 Berlin,. 23.-26. Juni 2003

43.4 43.6 43.8 44.0 44.2 44.4 44.61E-13

1E-12

sensitivity

peak positionS

igna

l/Ion

Cur

rent

amu

0 10 20 30 40 50 600

20

40

60

80

100

trend: aging of the channeltron

QMG 422 with Faraday Prisma with FaradayPrisma with Channeltron

Pa

rtia

l Pre

ssu

re S

en

sitiv

ity [

arb

uni

ts]

Time [hours]



change of the cracking pattern (due to the electron energy)

amu

Ar++



There are different instruments on the market which are used for various applications.

Whether the mass scale, the resolution, the fragmentation pattern and the amplification may drift and at which time scale rather depends on the design of the instrument and the quality of the electronics used to drive the instrument.

No general figures and numbers can be given for all these “RGA’s”

Anyway: Calibration is required because the ionization is gas specific.

Fragmentation leads to an overlap of different species at the same mass.

If the drift of the SEM or C-SEM is the major (only) source of instabilities, is there a method to get rid of these drifts?



The following considerations are valid for Quadrupole Mass Spectrometers, in which the drift of the amplifier is the major source of instabilities, the other’s are neglegible. (see EVC-8 in 2003)

Calibration:To minimize the effort for calibration, gas mixtures which contain several components in a chemical inert carrier gas are used. The pre-condition is that there is no overlap between the components contained in the mixture.Automatically one then gets the sensitivity of the components relative to the carrier gas and the cracking pattern of each component.

If the overall sensitivity of the instrument changes, then there is no influence on the result as long as the change occurs slow compared to a measurement cycle.

Furthermore if a calibration library exists with Argon and Nitrogen as carrier gas the sensitivity for Ar and for N2 have to be determined only in order to combine both libraries.



nature is not always kind!



Mass-Spectrum of CO2 recorded at an electron energy of 70 eV.



Model spectrum (Origin of peaks)

0 5 10 15 20 25 30 35 40 45 50

m/e

Hydrogen Nitrogen Oxygen Water

Carbon dioxide Argon Carbon monoxide



How to determine concentrations within a gas mixture of N2, CO and CO2 ??

a) Measure the concentration for CO2 at m/e = 44b) Subtract the contribution to mass 16 from CO2 (need to know the cracking pattern)c) Determine the concentration of CO at m/e = 16 then (hope there is no O2 in the sample!)d) Subtract the contribution of CO to the spectrum from the measured spectrum.e) Determine the concentration of nitrogen either at m/e=14 or at mass 28 now.



signal at mass 12 = PN2*SN2(12) + PCO*SCO(12) + PCO2*SCO2(12)



where PN2 = the pressure of N2 and SN2(12) is it’s sensitivity factor at mass 12 (which is zerofor mass 12) and so on for all the three components.

Three unknown and three equations.

Today’s computers solve such a system of linear equations in fractions of seconds.

Provided there are only the three components present and the sensitivity factors are preciselyknown, one get’s also a reasonable result.

However, here we still need to know the absolute sensitivity factors for the individual components.

Do we really?

Mathematics:



S12 SN2(12) SCO(12) SCO2(12) PN2

S14 = SN2(14) SCO(14) SCO2(14) X PCO

S16 SN2(16) SCO(14) SCO2(14) PCO2

The vector is the solution for the linear equation defined above.

PN2

PCO

PCO2

If we multiply the coefficients SN2, SCO and SCO2 above by a factor F, then

the vector 1/F X is a solution of the modified system of linear equations.

PN2

PCO

PCO2



Assume that the ratio of the individual sensitivity factors is precisely known,

then the correct ratio of the partial pressures will be achieved:

The result will be F*PN2, F*PCO and F*PCO2 with an undetermined factor F.

The assumption that only these three components are present in the gas leads to

[N2] + [CO] + [CO2] = 100%

which can be easily computed by the measured ratio of the partial pressures.



first have a look what’sthe residual gas composition



It’s not only the mass spectrometer whichdetermines the results.The vacuum system may be as important as well.

Subtract backgronund, however how stable is thebackground from the residual gas?



air – inlet and calibration by the known

composition of air:



still there is no overlap!



calibrate CO2 to Ar moreprecisely by using a gasmixture of known composition



In this way an existing calibration matrixcan be expanded.



Example: gas-mixture specified as

CO2 17.2%CO 18.5%Ar 19.2%

N2 45.1%

There is spectral overlap with this mixture.



Question?:re-calibrate with this mixturewas the previous mixture more precise?

pressure change within a measurement cycle

as there is no N2, CO, CO2, Ar in the sample, the assumptionthat the sum of all is 100% is not justified, result not applicable.



changes in process pressure do notinfluence the result

real, or due to changingbackground?



0 2 4 6 8 1090

92

94

96

98

100

The MID-Signal may be modulated by pressure fluctuations, thereforea short cycle time is an advatage.A too short dwell time however may lead to an inacurate measurement of

the ion current (10-12

ampere).

MID-Cycle: preselected masses are measured cyclic during a preprogrammed dwell time

Dwelltime Mass1 Dwelltime Mass2 ..... Dwelltime Mass n

MID-Cycle Time versus Pressure Fluctuations

+/- 1%

Tot

al P

ress

ure

[arb

. uni

ts]

time [arb. units]



proces pressure about 1*10-7 mbar, that’s too low for realisticmeasurement, because much to near to base pressurere-adjustment of background subtraction



How to check whether

re-calibration is required?



record a test-gas spectrumfrom time to time



Ar

Test-Gas-Mixture

export to excel



amu

Difference of two spectra, each normalized to 1000 at it’s maximum

difference +/- 3 % ok



amu

mass scale shift

difference of two spectra squared:



How to operate the instrument if the SEM or C-SEM is the major source of drift

spectral overlap of gaseous components lead to

MCD-Measurements (system of linear equations solved on line)

how to calibrate

examples

Suggestion how to check whether re-calibration is required

Apr 11, 202335 file_name


Thank you for your attention!

measurement of gas concentrations by mass spectrometers günter peter, n. müller, w. neff, bled...

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