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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 37 39 41 43 45 47 49
or hydrogen. Second, total pressure measurement is not very m/z
precisemeasurements better than 10 % are difficult and
expensive. If a system is operating at a standard pressure Figure 1: Partail pressure vs. mass
of 1.0