Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Appendix A
SRS Residual Gas Analyzer
3
dioxide shows a peak at 44 and a peaks for CO
2
++
and C
+
at 22 and 12. The other peaks
are caused by fragments of these species and contaminants.
The presence of air components in the spectra might lead us to believe that the system is
leaking, but this is untrue. The hybrid turbomolecular pump has simply reached its
compression limit. The foreline of the pump was operating at a total pressure of 0.5 Torr;
thereby the compression ratio is in the 10
8
range (as the pump specifications indicate).
Nitrogen, oxygen and argon are all present in the same ratios as standard atmosphere.
The presence of helium is interesting, because it is present in the atmosphere at about 7
ppm. Its peak might be expected 6 decades smaller than the nitrogen peak. The low
compression ratio of the turbo pump for helium (10
5
) explains why the peak is only three
decades smaller.
The ability to detect these common species and many others is the essence of the RGA.
The fragmentation of molecules in the ionizer of the RGA gives each molecule a distinct
fingerprint. The fragmentation patterns for many molecules is available from the library
in the SRS RGA program. Keeping a historical record of the typical spectrum of a
vacuum system allows the appearance of peaks to be instantly detected. For instance, the
peak at 48 in Figure 1 is SO from SO
2
(a matching peak at 64, not shown on the figure
confirmed this). On occasion this peak is seen in our chambers. If SO
2
appeared in your
vacuum system would you want to know? In addition to these simple gases, we are
interested in molecules with higher weights. The next two examples show how oil and
solvents can be detected.
Oil Contamination
Figure 2 contains a mass spectrum of a common contaminant of vacuum systems, oil.
The top pane is the measured spectrum and the bottom pane is the library data.