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Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, N. Müller, W. Neff, Bled April 2012 Quadrupole Mass Spectrometers are gas-specific.

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Presentation on theme: "Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, N. Müller, W. Neff, Bled April 2012 Quadrupole Mass Spectrometers are gas-specific."— Presentation transcript:

1 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

2 Measurement of Gas Concentrations by Mass Spectrometers 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.

3 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 Ion-Source 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.

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

5 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 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, Juni 2003

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

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

8 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 The following considerations are valid for Quadrupole Mass Spectrometers, in which the drift of the amplifier is the major source of instabilities, the others 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 N 2 have to be determined only in order to combine both libraries.

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

10 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 Mass-Spectrum of CO 2 recorded at an electron energy of 70 eV.

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

12 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 How to determine concentrations within a gas mixture of N 2, CO and CO 2 ?? a)Measure the concentration for CO 2 at m/e = 44 b)Subtract the contribution to mass 16 from CO 2 (need to know the cracking pattern) c)Determine the concentration of CO at m/e = 16 then (hope there is no O 2 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.

13 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 signal at mass 12 = P N 2 *S N 2 (12) + P CO *S CO (12) + P CO 2 *S CO 2 (12) signal at mass 14 = P N 2 *S N 2 (14) + P CO *S CO (14) + P CO 2 *S CO 2 (14) signal at mass 16 = P N 2 *S N 2 (16) + P CO *S CO (16) + P CO 2 *S CO 2 (16) where P N 2 = the pressure of N 2 and S N 2 (12) is its sensitivity factor at mass 12 (which is zero for mass 12) and so on for all the three components. Three unknown and three equations. Todays 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 gets also a reasonable result. However, here we still need to know the absolute sensitivity factors for the individual components. Do we really? Mathematics:

14 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 S12 SN 2 (12) SCO(12) SCO 2 (12) PN 2 S14 = SN 2 (14) SCO(14) SCO 2 (14) X PCO S16 SN 2 (16) SCO(14) SCO 2 (14) PCO 2 The vector is the solution for the linear equation defined above. P N 2 P CO P CO 2 If we multiply the coefficients S N 2, S CO and S CO 2 above by a factor F, then the vector 1/F X is a solution of the modified system of linear equations. P N 2 P CO P CO 2

15 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 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*P N 2, F*P CO and F*P CO 2 with an undetermined factor F. The assumption that only these three components are present in the gas leads to [N 2 ] + [CO] + [CO 2 ] = 100% which can be easily computed by the measured ratio of the partial pressures.

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

17 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 Its not only the mass spectrometer which determines the results. The vacuum system may be as important as well. Subtract backgronund, however how stable is the background from the residual gas?

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

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

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

21 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 calibrate CO 2 to Ar more precisely by using a gas mixture of known composition

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

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

24 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 Example: gas-mixture specified as CO % CO 18.5% Ar 19.2% N2 45.1% There is spectral overlap with this mixture.

25 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 Question?: re-calibrate with this mixture was the previous mixture more precise? pressure change within a measurement cycle as there is no N 2, CO, CO 2, Ar in the sample, the assumption that the sum of all is 100% is not justified, result not applicable.

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

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

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

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

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

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

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

33 Measurement of Gas Concentrations by Mass Spectrometers Günter Peter, Bled April 2012 amu mass scale shift difference of two spectra squared:

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

35 June 1435file_name

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


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