Inficon Transpector XPR 3+ - 4.1.2 Quantitative Interpretation of Mass Spectra (Calculating Partial Pressures)

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Transpector XPR 3+ Operating Manual
4.1.2 Quantitative Interpretation of Mass Spectra
(Calculating Partial Pressures)
Partial pressure is defined as the pressure of a designated component in a gas
mixture. By Dalton’s Law, the sum of all the partial pressures is the total pressure.
The partial pressure analyzer is designed so that the height of a peak in a mass
spectrum is proportional to the number of ions giving rise to that peak. Also by
design, the number of ions is more or less proportional to the partial pressure of the
substance giving rise to that peak (over some specified operating pressure range).
Therefore, the height of a peak is proportional to the partial pressure of the
substance giving rise to that peak.
The following equation shows the relationship between the partial pressure of
substance determined by measuring the ion current at mass b:
[1]
The partial pressure of substance a is symbolized by PP
a
, while K
ab
is the
proportionality constant for the peak at mass b from substance a, and I
ab
is the ion
current at mass b from substance a.
The proportionality constant, K
ab
, depends on the nature of the substance being
detected and on the characteristics of the partial pressure analyzer. The
substance-dependent part is called the material factor, M
ab
. The
instrument-dependent part is called the analyzer factor, A
b
, and depends primarily
on the ion mass, b. Therefore, the original equation [1] can therefore be rewritten
as follows:
[2]
49 C
37
Cl See Note 2
50 C
37
Cl, CF
2
, C
4
H
2
See Note 2, Freon F, Note 3
NOTES: (1) Fragments of several hydrocarbons, such as mechanical pump oil,
diffusion pump oil, vacuum grease, cutting oil, and organic solvents.
(2) Fragments of several chlorinated hydrocarbons, such as carbon
tetrachloride, tichloroethylene, and many freons.
(3) Fragments from both straight chain hydrocarbons and benzene ring
hydrocarbons.
(4) F = Fragment ion; P = Parent ion; DI = Doubly ionized
Table 4-3 Spectrum Interpretation Guide (continued)
Spectrum Interpretation Guide
AMU # Chemical Symbol Sources
PP
a
K
ab
I
ab
=
PP
a
M
ab
A
b
I
ab
=