Stanford Research Systems RGA100, Operating Manual And Programming Reference

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2-12 Residual Gas Analysis Basics
SRS Residual Gas Analyzer
The underlying assumption when using sensitivity factors in quantitative calculations is that there
is a linear relation between the partial pressure and the corresponding RGA signals of the gases.
Deviations from linearity are to be expected above 10
-5
Torr due to space charge effects in the
ionizer and ion-neutral scattering interactions in the filter. A more thorough check of the RGA’’s
sensitivity involves measuring the RGA signals over several orders of magnitude of partial
pressure to determine the range over which a linear relationship exists. The sensitivity factor for
the gas is calculated as the slope of the ““signal vs. partial pressure”” response over the linear
range.
RGA Windows uses two sensitivity factors stored in the non-volatile memory of the RGA Head.
The sensitivity factors, one for total pressure and one for partial pressure, are used as conversion
factors between the ion currents received form the head and the pressure units selected by the
user. The sensitivity factors are measured with the Faraday Cup detector and can be updated or
changed very easily using the Sensitivity Tuning command in the Head menu. A separate
Electron Multiplier Gain Factor, stored in the non-volatile memory of the RGA Head, is used to
correct the ion signals for the gain of the electron multiplier. The gain of the electron multiplier is
highly mass dependent and defined relative to the corresponding FC signal. An automatic
Electron Multiplier Gain Adjustment command, built into the program, can adjust the CDEM
voltage for any gain between 10 and 10
6
. Consult the RGA On-Line Help Files for details on the
automated tuning procedures built into the RGA Windows program. Also see the Sensitivity and
Electron Multiplier Tuning sections of the RGA Tuning Chapter for more general information.
The Table mode of RGA Windows offers scaling factors for all of its channels eliminating the
limitations imposed by the single sensitivity factor on multiple partial pressure calculations. For
example, the scaling factors can be used to display correct partial pressure for all the species in a
table if the ratios between the partial pressure sensitivities of the different components are known
and only principal mass peaks are used to monitor them. The scaling factors can also be adjusted
to correct against the mass discrimination of the electron multiplier’’s gain.
Important : Following current industry standards, the partial pressure sensitivity factor stored at
the factory corresponds to N
2
measured at 28 amu with
'
m
10%
=1 amu, default ionizer settings
and Faraday Cup Detection.
Single gas measurement example:
Monitoring the concentrations of one or few components in a system is easy in the absence of
severe spectral interference.
Suppose a system where argon is measured at 40 amu (principal mass), in the absence of any
other gases that contribute a signal at that mass value. The sensitivity to argon was previously
measured at
S
Ar
=10
-4
amps/Torr, and the electron multiplier is biased and its gain at mass 40 was
previously measured at
g
CDEM
=1.02
.
10
3
relative to the FC signal.
The partial pressure of argon,
P
Ar
, is easily calculated measuring the intensity (i.e. peak height)
of the ion current at mass 40,
I
40
.
P
Ar
=
I
40
/ (
g
CDEM
S
Ar
) , units of Torr (4)
The peak value,
I
40
, can be extracted from a spectral scan or measured directly using the single
mass measurement mode of the SRS RGA. For example, a 10
-9
amp peak value corresponds to