SRS Labs QMS 100 Series, User Manual

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1–16 Mass Spectrometry Basics
SRS QMS Gas Analyzer
Mass Spectrometry Basics
The RGA can perform both qualitative and quantitative analysis of the gases in a vacuum system.
Obtaining spectra with the RGA is very simple. Interpreting the spectra, that is, understanding what the
spectra is trying to tell you about your vacuum system requires some work. The following sections will
introduce some basic concepts of Spectral Analysis emphasizing the main aspects of Residual Gas
Analysis. For additional information on the subject of Residual Gas Analysis refer to:
J. Drinkwine and D. Lichtman, Partial Pressure Analyzers and Analysis, AVS
Monograph Series published by the Education Committee of the American Vacuum
Society
Basford et. al., J. Vac. Sci. Technol. A 11(3) (1993) A22-40 “Recommended
Practice for the Calibration of Mass Spectrometers for Partial Pressure Analysis.
Update to AVS Standard 2.3”.
For information on multiple linear regression analysis consult:
William H. Press, et. al., 1992, Numerical Recipes in C, The Art of Scientific
Computing, Second Edition, Cambridge Univ. Press, section 15.4, page 671.
Bevington, P.R., 1969, Data Reduction and Error Analysis for the Physical Sciences,
New York, McGraw-Hill, Chapters 8-9.
How Mass Spectra are Interpreted
A mass spectrum, taken in a real system, will almost always contain signals from a mixture of various
gases. Careful and complete interpretation of the spectrum (i.e. a complete spectral analysis) should
reveal the identity, as well as the concentrations, of the various components which have produced the
spectrum.
The first step in the spectral analysis process is to correctly identify the mass-to-charge ratio of all the
peaks in the mass spectrum. A well calibrated mass scale is essential to this task. See the RGA Tuning
Chapter for a detailed description of the mass scale calibration procedure.
Once all the peaks have been labeled, the next step is to identify the residual gases that have produced
the spectrum. A knowledge of the recent history of your system may provide very valuable clues as to
the possible gases that may be residuals in the vacuum chamber. A familiarity with the standard spectra
of commonly expected gases will generally help to determine the major and minor components in the
system. Any peak in the spectrum may consist of contributions from molecular ions and/or fragment
ions, or multiply ionized species.
The qualitative spectral analysis is completed when all the
peaks in the spectrum have been “uniquely assigned” to the components of a gas mixture, in
complete agreement with the known fragmentation patterns of the components.
In cases where only the major components are of interest, some of the minor peaks of the spectrum will
remain unassigned. If only a few species are being monitored, only the peaks corresponding to the
substances of interest need to be assigned and monitored.