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                               By Hoongsun Im and Jim Lee, R&D Engineers, Standford Research Systems, Sunnydale, CA

                                                                          sample valve open, while the blue traces show the response with the

R
         esidual gas analyzers (RGAs) have long been used to charac-
         terize the components of a gas mixture. They are based on a      valve closed. In the single DP configuration, the RGA still picks up sig-
         quadrupole mass spectrometer with a hot filament ionizer. A      nificant concentrations of hydrogen from backstreaming. However, in
turbomolecular pump (TP) is commonly used to generate the high vac-       the dual DP configuration, when the sample valve is closed the hydro-
uum region (<10-5 Torr) required to operate the ionizer and quadrupole.   gen signal drops to a level consistent with internal degassing. The data
More recently, a variety of applications for atmospheric sampling were    show the backstreaming problem is virtually eliminated in the dual DP
developed including fuel cell research, Freon detection, specialty gas    system employed by the UGA.
production monitoring, fermentation process monitoring, and catalysis
studies. To accommodate atmospheric sampling, a pressure reduction        Feature Highlights
scheme is required. Most commercially available systems employ a             The RGA chamber is a six-way cross with 2-3/4" conflat flanges.
capillary tube, pinhole, and single diaphragm pump (DP) to reduce the     Two ports are available for direct connection to various vacuum regions
pressure exposed to the RGA. The DP performs double duty, evacuat-        (from 0.1 Torr to 10 -7 Torr) using the proper connection, or the user can
ing a bypass line and backing the TP. The drawback to this architecture   use the built-in capillary pressure reduction system to sample atmos-
is that the DP will allow some of the gas in the bypass line to back-     pheric pressures (See Figure 3). This capability makes it possible to
stream through the TP. As discussed below, this backstreaming can         use the UGA as a helium leak detector or for other "sniffing" applica-
cause real problems for users measuring corrosive or low molecular        tions. Several heaters (standard equipment) cover the sample inlet,
weight gases.                                                             chamber, and the connection to the TP. Bakeouts up to 120

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