Stanford Research Systems RGA100, Operating Manual And Programming Reference

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2-8 Residual Gas Analysis Basics
SRS Residual Gas Analyzer
Residual Gas Analysis Basics
The SRS RGA can perform both qualitative and quantitative analysis of the gases in a
vacuum system. Obtaining spectra with the SRS 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. Consult Appendix A of this
manual for specific examples on the application of the SRS RGA to vacuum diagnosis
For additional information on the subject of Residual Gas Analysis refer to:
1. J. Drinkwine and D. Lichtman, Partial Pressure Analyzers and Analysis, AVS
Monograph Series published by the Education Committee of the American Vacuum
Society
2. 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:
1. 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.
2. 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.