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Transpector MPH 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.
Transpector MPH 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.
43 C
3
H
7
, CH
3
CO Note 1, acetone F or methyl ethyl ketone F
44 CO
2
, C
3
H
8
carbon dioxide, See Note 3
45 CH
3
CH
2
O ethanol F or Isopropyl alcohol F
46 CH
3
CH
2
OH ethanol P
47 C
35
Cl See Note 2
48 HC
35
Cl, SO See Note 2, sulfur dioxide F
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
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