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Transpector SPS Operating Manual
5.3.2 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. 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.
shows the relationship between the partial pressure of substance a
determined by measuring the ion current at mass b:
[4]
PP
a
= the partial pressure of substance a
K
ab
= the proportionality constant for the peak at mass b from substance a
I
ab
= 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
can therefore be rewritten as follows:
[5]
The material factor (M
ab
) depends on the fragmentation pattern for the particular
substance, the fragmentation pattern for a reference gas (usually nitrogen), and the
ease with which the substance can be ionized relative to the same reference gas.
The relationship is shown in
:
[6]
The fragmentation factor for substance (a) at mass (b) is (FF
ab
). It is equal to the
fraction of the total current of all ions from substance (a) which have a mass (b).
The ionization probability of substance (a), relative to nitrogen (i.e., XF
N
=1) is
(XF
a
). That is, it is the ratio of total ion current (for all masses) from substance (a)
to the total ion current from nitrogen (N), both measured at the same true partial
pressure. Both fragmentation factors and ionization probabilities depend strongly
on the energy of the ionizing electrons. If the correct values of these factors are not
known for the exact conditions of the particular analyzer being used, they can be
approximated using published values for other conditions with, generally, only a
small loss in accuracy.
PPa Kab Iab
=
PPa
Mab Ab
Iab
=
Mab
1
FFab XFa
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=
