Stanford Research Systems RGA100, Operating Manual And Programming Reference

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6 Appendix B
SRS Residual Gas Analyzer
Glass capillaries are available with small enough bores to reduce pressure from
1000 mbar to 10
-6
mbar without bypass pumping. While it is possible to build a
atmospheric sampling system based on a 1/4 meter 50
P
m glass capillary, there are
considerable reasons to use a bypass pump configuration. Bypass pumping improves the
operation of a system by increasing the flowrate of gas through the capillary about 3-4
orders of magnitude. The higher flowrates and smaller pressure drop allow a wider
selection of capillaries to be practical. Stainless steel and PEEK capillaries are more
affordable and flexible than glass capillaries. A large flowrate means that the volumetric
flowrate at the inlet of the capillary is more reasonable. For a system with 70 liter s
-1
pumping speed, operating at 10
-6
mbar, the volumetric flowrate at the inlet would be 70
nanoliter s
-1
. Any dead volume at the inlet of the capillary would result in an
unreasonable response time. With such small flowrates, inlet devices such as filters,
valves, or connecting hardware cannot be used. Overall, the bypass pumped capillary
system is more flexible and only requires a minor addition of hardware (one valve and
some tube).
The configuration seen in Figure 4 is made possible by the recent advances in hybrid
turbomolecular/drag pumps and diaphragm pumps. Traditional designs would have
relied on a
two rotary vane pumps and standard turbomolecular pump. The high
compression ratios of the hybrid turbo pumps allow the two streams (bypass and sample)
to be combined. The low ultimate vacuum of contemporary diaphragm pumps makes
them suitable as a foreline pump. The combination of these modern technologies means
that an atmospheric sampling system can be constructed into a very small packages (less
than 8 inch high in a 19 inch rack mount chassis), which is portable and easy to operate.
Conclusion
Although the RGA is intrinsically a vacuum instrument, inlet systems are easily designed
that allow it to sample gasses at any pressure. A more descriptive name for such systems
would be ““online quadrupole mass spectrometer””. Mass spectrometry is a well proven
analytical technique, but traditionally used an expensive large machine. Reduction in
cost of quadrupoles and vacuum pumps, along with the development of easy to use
software interfaces makes process analysis with mass spectrometry an attractive
technique.
Further Reading
Lewin, O., An Elementary Introduction to Vacuum Technique , American Vacuum
Society, New York, 1987.
Moore, J.H., Davis, C.C., and Coplan, M.A., Building Scientific Apparatus , 2nd ed. ,
Addison-Wesley, New York, 1989.