Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Appendix A
SRS Residual Gas Analyzer
5
The presence of mechanical pump oil is immediately obvious. The peaks at masses 39,
41, 43, 55, and 57 are caused by mechanical pump oil backstreaming into the vacuum
chamber during a load lock sequence. The total pressure in the chamber was dominated
by water and was less than 2
u
10
-8
Torr. In this case, the total pressure might satisfy
operating conditions but the spectra reveals that the system is heavily contaminated with
oil. This could have been caused by improper valve sequencing or a saturated oil trap.
Without an RGA, only operating procedures for valves and a maintenance schedule for
traps can ensure that the cleanliness of a vacuum system is maintained. With the RGA,
the cleanliness can be
guaranteed before a process or experiment begins.
Solvent Contamination
Oil contamination is common in vacuum systems. Cleaning parts with solvents is a
common approach to removing this contamination. We have observed that organic
solvents such as acetone and 1,1,1-trichloroethane (TCE) are more tenacious
contaminants than the oil they are designed to remove. Figure 3 shows the spectrum of a
vacuum system contaminated with TCE as evidenced by the major peaks at 97 and 99,
and the minor peaks at 61, 63, 117 and 119. The paired peaks are caused by the natural
isotopic occurrence of
35
Cl and
37
Cl (75% and 25%). This spectrum was measured one
week after the initial contact with the solvent. The TCE permeated into the o-rings in the
system during a cleaning step. The TCE continued to outgas from the o-rings for two
weeks and showed no signs of stopping. At that time they were removed and baked in an
oven, which eventually removed the TCE.
1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121
-11
1.0x10
-10
1.0x10
-9
1.0x10
-8
1.0x10
-7
1.0x10
amu
Torr
TCE Contaminated System
Figure 3: TCE Contaminated System
Data like this is invaluable to the development of cleaning procedures. While the TCE
successfully removed the oil, the vacuum chamber was left more contaminated than it