Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 43 / 362

Residual Gas Analysis Basics 2-11
SRS Residual Gas Analyzer
Partial Pressure Sensitivity Factors
The partial pressure sensitivity of the RGA to a gas g,
S
g
, is defined as the ratio of the
change
(H-H
0
)
in principal mass peak height to the corresponding change
(P-P
0
)
in
total pressure due to a change in partial pressure of the particular gas species.
H
0
and
P
0
are background values.
S
g
= (H-H
0
) / (P-P
0
)
The units of
S
g
are of ion current per unit pressure (amp/Torr, for example).
The sensitivity of the RGA varies with different gases, changes with time due to aging of
the head, and is a strong function of the operating conditions of the instrument. Careful
quantitative analysis requires that the sensitivity factor,
S
g
, be determined for every gas
which may be a component gas in the system being analyzed. The sensitivity factors must
be obtained under the same operating conditions that will be used during general partial
pressure analysis since they depend on many instrumental parameters, including:
ionization energy, emission current, mass filter setting, type of detector, etc.
In order to separate the gain of the electron multiplier from the intrinsic sensitivity of the
RGA head, the sensitivity factors of the SRS RGA are defined for Faraday Cup detection.
A separate Electron Multiplier Gain Factor, is used to correct the ion signals when the
electron multiplier is turned on. See the Sensitivity and Electron Multiplier Tuning
sections of the RGA Tuning Chapter for details.
The basic procedure for determining the sensitivity of a particular gas in the RGA is the
following:
x
Introduce the pure gas into the vacuum system, at a known or calculable pressure
(typically around 10
-6
Torr).
x
Measure the output signal from the RGA for the principal mass peak of that gas
using the Faraday cup detector.
x
The ratio of this output signal to the pressure of the gas is the sensitivity factor,
S
g
.
During these measurements it is very important to insure that the partial pressures of all
other gases in the system are small enough so that they may be neglected. The sensitivity
factors calculated can only be applied to situations where the RGA is used with the same
operating parameters. See the Sensitivity Tuning section in the RGA Tuning chapter of
this manual for more details on this calibration procedure.
A total pressure sensitivity factor is also needed by the SRS RGA to convert the ion currents
obtained during total pressure measurements into total pressures. Total pressure sensitivity factors
vary with different gases and share many of the properties of the partial pressure factors. They are
determined by a procedure identical to the one described above, but with the partial pressure
measurements replaced by total pressure measurements.