Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Residual Gas Analysis Basics 2-9
SRS Residual Gas Analyzer
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.
Notes on Fragmentation Patterns : The electron impact type of ionizer used in modern
RGA’’s almost always causes more than one kind of ion to be produced from a single type
of gas molecule. Multiple ionization, molecular fragmentation and changes in the isotopic
composition of the molecule are responsible for the effect. All ions formed contribute to
the mass spectrum of the molecule and define its
fragmentation pattern . The
identification and interpretation of mass spectra must begin with a knowledge and
understanding of the standard fragment patterns of atoms and molecules that may exist in
the system. The standard fragment patterns of most molecules commonly encountered in
residual gas analysis are well established and listed in the general RGA Literature. A very
complete library can also be accessed through the Library Search Utility of the RGA
Windows software. The Gas Library has a standard text file format, and can easily be
read, extended or modified by the user to fit his individual needs.
Residual Gas Analysis Tip: Virtually every vacuum system will have detectable
amounts of hydrogen (2 amu), water (18 amu), carbon monoxide (28 amu) and carbon
dioxide (44 amu). Become familiar with these species and their fragmentation patterns,
and use their peaks to verify the correct performance of the instrument (i.e. mass scale
calibration and mass resolution) while operating the RGA.
Partial Pressure Measurement
Once the different components of a mixture have been identified it is possible to use the
SRS RGA to obtain quantitative values for the various partial pressures. This section
describes the basic steps needed to perform quantitative measurements with the
instrument. The formalism presented assumes multiple gas analysis, but is equally valid
for single gas measurements. Please consult the suggested references for details and
examples of these procedures.
The entire mathematical formalism used to derive the partial pressures of a mixture based
on a single mass spectrum is based on one assumption:
The total spectrum is a linear combination of the spectra of the different species that
are present in the mixture
. In other words, the total spectrum is equal to the sum of the
individual peaks that would be observed if each constituent were alone in the system.
In mathematical terms, the assumption stated above can be written as the following linear
equation:
H
M
=
6
g
h
Mg
(1)
where:
g
is an integer variable that represents the gases present (i.e. assign an integer to
each gas starting with one)
M
is an integer variable that represents the mass numbers for the entire mass
range of the spectrum.