SRS Labs QMS 100 Series, User Manual

Page: 28 / 280

1–18 Mass Spectrometry Basics
SRS QMS Gas Analyzer
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
=
Σ
g
(S
g
α
M g
) 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
α
M g
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
α
M g
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
α α
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 RGA to different gases as shown below.
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).