4 - 15
Transpector XPR 3+ Operating Manual
XF
a
. . . . . . . . . . . . . . . . . . . . . . . . . . Ionization probability of substance
a
relative
to nitrogen; approximately the same as the
relative ion gauge sensitivity as shown in
(dimensionless).
TF
b
. . . . . . . . . . . . . . . . . . . . . . . . . . Transmission factor, the fraction of total ions
at mass b which pass through the mass filter,
relative to ions with a mass of 28 AMU;
nominally,
T
FM
= 28 / M
(dimensionless).
DF
ab
. . . . . . . . . . . . . . . . . . . . . . . . . Detection factor for mass
b
ions from
substance
a
, relative to nitrogen at 28 AMU;
assumed to be 1.00 for Faraday detectors,
but varies for Electron Multiplier detectors
(dimensionless).
G
. . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron Multiplier gain for nitrogen ions at 28
AMU (dimensionless; set equal to 1 for a
Faraday Cup detector).
S
. . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensitivity of instrument to nitrogen, the ion
current at 28 AMU per unit of nitrogen partial
pressure (usually in amps/Torr).
I
ab
. . . . . . . . . . . . . . . . . . . . . . . . . . . Ion current of mass peak
b
resulting from
substance
a
(in amps; assumes that there are
no other substances present which
contribute significantly to the total current at
mass peak
b
).
4.1.3 Additional Information for Interpreting Mass Spectra
The following paragraphs contain additional information which may be of use when
interpreting mass spectra.
4.1.3.1 Ion source Characteristics
It is important to recognize that the partial pressure analyzer (especially the ion
source) and the vacuum system configuration can both have an effect on the
relative concentrations of the gases detected. In order to minimize these effects,
it is necessary to have the right type of ionizer, the right type of filament, and the
right configuration of the vacuum system. This is particularly true when a
differential pumping arrangement is used because the pressure of the gas to be
sampled is too high for the Sensor to operate. J. O’Hanlon’s book,
A User’s Guide
to Vacuum Technology
, has a brief discussion (in Chapter 8, Section 2) of some
of these concerns.
