Stanford Research Systems RGA100, Operating Manual And Programming Reference

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2-10 Residual Gas Analysis Basics
SRS Residual Gas Analyzer
H
M
= total peak height (amps) of the spectrum at mass number M.
h
Mg
= peak height contribution (amps) from gas g at mass M.
h
Mg
is related to the fragmentation pattern, the RGA’’s sensitivity and the partial pressure of gas g
by the equation:
h
Mg
=
D
Mg
S
g
P
g
(2)
where:
D
Mg
= Fragmentation factor of gas g at mass M: Ratio of ion signal at mass M to the ion
signal at the principal mass peak for gas g.
S
g
= RGA’’s partial pressure sensitivity factor for gas g, in amp/Torr (see Partial Pressure
Sensitivity Factor below)
P
g
= Partial pressure of gas g in the system.
Equations (1) and (2) are combined to obtain the system of equations:
H
M
=
6
g
(S
g
D
Mg
) P
g
(3)
Since all gases have more than one peak in their fragmentation pattern, the number of peaks (M)
in a real spectrum is generally larger than the number of gases (g). As a result, the system of
equations (3) usually has more equations than unknowns. This situation is sometimes simplified
eliminating some of the extra equations; however, the best results are obtained using all the
equations and a multiple linear regression procedure to calculate the best possible fit to the data.
Obviously, accurate results can only be obtained if the constants
D
Mg
and
S
g
are well known for
the RGA being used.
Note: The Analyze Utility of RGA Windows uses a multiple linear regression algorithm, as
mentioned above, to automatically calculate the composition of a ““typical residual gas
environment”” at the end of any 1-50 amu spectral scan. Please see the RGA On-Line Help files
for details.
Standard fragmentation patterns (for example, the fragmentation patterns included in the RGA
Library of RGA Windows) can be used as a source of
D
Mg
values in moderately quantitative
determinations. However, when very precise numbers are desired, one should obtain the
appropriate fragment patterns by introducing pure gas into the RGA being used. The fragment
patterns must be obtained under the same conditions that will be used during regular spectral
analysis since they depend on many instrumental parameters, including: electron energy,
emission current, ionizer design, mass filter settings, detector type, multiplier gain, etc. The
principal mass peak of a fragmentation pattern is simply the most intense peak of the spectrum,
and the intensity of all the other peaks in the pattern are normalized to its height for the
calculation of fragmentation factors.
Note that by our definition the
D
Mg
value for the
principal mass peak of any gas is equal to one. Principal mass peaks are used in the calculation
of the sensitivity of the SRS RGA to different gases as shown below.