Measurement of Gas Concentrations by Mass Spectrometers

1. 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 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, N. Müller, W. Neff, Bled April 2012

2. Functional units of every Quadrupole Mass-Spectrometer: Measurement of Gas Concentrations by Mass Spectrometers An ion-current, amplified ion-current or a count rate is the original out-put of these instruments. Günter Peter, Bled April 2012

3. Ion-Source Measurement of Gas Concentrations by Mass Spectrometers The amount of ions generated in the ion source depends on the partial pressure of the 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. Günter Peter, Bled April 2012

4. IonizationProbabilitycomparedtonitrogen(electronenergy : 70 eV) Measurement of Gas Concentrations by Mass Spectrometers The same current for He and Ar makes a difference in pressure of a factor of 8. Günter Peter, Bled April 2012

5. 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. Measurement of Gas Concentrations by Mass Spectrometers * 8th European Vacuum Congress EVC-8 Berlin,. 23.-26. Juni 2003 Günter Peter, Bled April 2012

6. change of the cracking pattern (due to the electron energy) Measurement of Gas Concentrations by Mass Spectrometers Ar++ amu Günter Peter, Bled April 2012

7. 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? Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

8. 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. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

9. nature is not always kind! Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

10. Mass-Spectrum of CO2 recorded at an electron energy of 70 eV. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

11. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

12. How to determine concentrations within a gas mixture of N2, CO and CO2 ?? Measurement of Gas Concentrations by Mass Spectrometers • Measure the concentration for CO2 at m/e = 44 • Subtract the contribution to mass 16 from CO2 (need to know the cracking pattern) • Determine the concentration of CO at m/e = 16 then (hope there is no O2 in the sample!) • Subtract the contribution of CO to the spectrum from the measured spectrum. • Determine the concentration of nitrogen either at m/e=14 or at mass 28 now. Günter Peter, Bled April 2012

13. Mathematics: signal at mass 12 = PN2*SN2(12) + PCO*SCO(12) + PCO2*SCO2(12) signal at mass 14 = PN2*SN2(14) + PCO*SCO(14) + PCO2*SCO2(14) signal at mass 16 = PN2*SN2(16) + PCO*SCO(16) + PCO2*SCO2(16) where PN2 = the pressure of N2 and SN2(12) is it’s sensitivity factor at mass 12 (which is zero for 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 precisely known, 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? Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

14. 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 PN2 PCO PCO2 Measurement of Gas Concentrations by Mass Spectrometers The vector is the solution for the linear equation defined above. 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 Günter Peter, Bled April 2012

15. 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. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

16. first have a look what’s the residual gas composition Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

17. Measurement of Gas Concentrations by Mass Spectrometers It’s not onlythe mass spectrometerwhich determinestheresults. The vacuumsystemmaybeasimportantaswell. Subtractbackgronund, howeverhowstableisthe backgroundfromthe residual gas? Günter Peter, Bled April 2012

18. air – inlet and calibration by the known composition of air: Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

19. still there is no overlap! Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

20. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

21. calibrate CO2to Ar more preciselybyusing a gas mixtureofknowncomposition Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

22. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

23. In this way an existing calibration matrix can be expanded. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

24. Example: gas-mixturespecifiedas CO2 17.2% CO 18.5% Ar 19.2% N2 45.1% Measurement of Gas Concentrations by Mass Spectrometers Thereisspectraloverlapwiththismixture. Günter Peter, Bled April 2012

25. Measurement of Gas Concentrations by Mass Spectrometers as there is no N2, CO, CO2, Ar in the sample, the assumption that the sum of all is 100% is not justified, result not applicable. pressure change within a measurement cycle Question?: re-calibrate with this mixture was the previous mixture more precise? Günter Peter, Bled April 2012

26. changes in process pressure do not influence the result Measurement of Gas Concentrations by Mass Spectrometers real, or due to changing background? Günter Peter, Bled April 2012

27. Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

28. proces pressure about 1*10-7 mbar, that’s too low for realistic measurement, because much to near to base pressure re-adjustment of background subtraction Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

29. How to check whether re-calibration is required? Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

30. record a test-gas spectrum from time to time Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

31. Ar Test-Gas-Mixture Measurement of Gas Concentrations by Mass Spectrometers export to excel Günter Peter, Bled April 2012

32. Difference of two spectra, each normalized to 1000 at it’s maximum difference +/- 3 % ok Measurement of Gas Concentrations by Mass Spectrometers amu Günter Peter, Bled April 2012

33. differenceoftwospectrasquared: mass scale shift Measurement of Gas Concentrations by Mass Spectrometers amu Günter Peter, Bled April 2012

34. 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 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012

35. file_name

36. Thank you for your attention! Günter Peter, Bled April 2012