Specifications

2 Appendix A
SRS Residual Gas Analyzer
Composition Analysis
The SRS RGA software allows the composition of the vacuum system to be analyzed by
two methods. The most common is to measure the mass spectrum of the vacuum. This
provides a “fingerprint” of the residual gases in the vacuum system. A second method is
to track specific species or peaks of the mass spectrum. The first method, analog scan
mode, is most useful when the user does not know what is present in the chamber. Once
the identities of the species have been determined, individual peaks can be tracked using
either pressure vs. time, table or annunciator mode.
The Mass Spectrum
The fundamental operation of the RGA is as a mass spectrometer. Figure 1 shows a
graph of partial pressure versus mass, which was measured with an RGA with an electron
multiplier detector.
1 3 5 7 9 11131517 1921 2325 2729 3133 3537 3941 4345 4749
-13
1.0x10
-12
1.0x10
-11
1.0x10
-10
1.0x10
-9
1.0x10
-8
1.0x10
-7
1.0x10
m/z
Torr
Ultimate Vacuum
Figure 1: Partial Pressure vs. Mass
The scan was taken of a vacuum system near its ultimate vacuum. The pressure axis is
plotted on a logarithmic scale so that a large range can be seen. The log scale makes the
peaks appear wider than when plotted on a linear scale. This scan from 1 to 75 amu
shows some gases commonly present in vacuum chambers. There are many peaks, but
they are caused mainly by 7 species: Hydrogen is at 2 and helium at 4. Water gives
primary peaks at 16,17, and 18 due to the species O
+
, HO
+
, and H
2
O
+
. The smaller peaks
at 19 and 20 are due to
18
O which is naturally present at 0.2%. Nitrogen is at 28 and also
causes the peaks at 14 by atomic N
+
and the doubly ionized N
2
+
. Molecular oxygen
shows a peak at 32 and an isotope peak at 34. Argon shows a peak at 40. Carbon