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www.thinkSRS.com Vacuum Diagnosis with an RGA Application Note #7 Residual Gas Analyzer (RGA) is the term used for a class of The Mass Spectrum quadrupole mass spectrometers that typically cover mass ranges from 1 to 100 or 200 amu (atomic mass units), and are The fundamental operation of the RGA is as a mass intended to be used for the analysis of the gasses present in spectrometer. Figure 1 shows a graph of partial pressure high and ultra-high vacuum. The RGA's resolution is versus mass, which was measured using an RGA with an sufficient to clearly distinguish peaks that are 1 amu apart. electron multiplier detector. The scan is of a vacuum system These specifications are a perfect match for the requirements near its ultimate vacuum. The pressure axis is plotted on a of vacuum diagnosis. Not many materials with a mass logarithmic scale so that a large range can be seen. The log greater than 200 amu will be volatile, and the high resolution scale makes the peaks appear wider than when plotted on a of research grade mass spectrometers is not necessary for the linear scale. analysis of low molecular weight species. Overall, RGAs are affordable instruments that can be permanently attached to a Ultimate Vacuum vacuum system. Torr -7 1.0x10 The purpose of vacuum is to remove molecules that would -8 interfere with a process or experiment. Although the reduction 1.0x10 of total pressure is always a concern in vacuum systems, near 1.0x10 -9 operating pressure, the real concern becomes the presence of certain species: oxygen, water, and hydrocarbons. When 1.0x10 -10 operating a vacuum system with only a total pressure vacuum gauge, one must rely on the assumption that the total pressure 1.0x10 -11 is a direct indicator of the partial pressure of these crucial impurities. This assumption presents two problems. First, a 1.0x10 -12 total pressure measurement cannot tell the user whether the vacuum system is filled with water, nitrogen, carbon dioxide 1.0x10-13 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35