5–31
Transpector CPM Operating Manual
The overall relation between partial pressure and ion current, given in equation [8],
is quite general. The constants for this equation can be obtained from various
tables, but for the best accuracy, they should be measured for each instrument.
[8]
PP
a
= partial pressure of substance a (usually in Torr).
FF
ab
= fragmentation factor, or fraction of total ion current from substance a having mass b
(dimensionless; see Table 5-8 on page 5-28).
FF
N28
= fragmentation factor for N
2+
ions at 28 AMU from nitrogen (dimensionless; typically
around 0.9).
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
a
b
= 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 EM detectors (dimensionless).
G = EM 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 A/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).
PP
a
FF
N28
FF
ab
XF
ab
TF
b
DF
ab
G S
-------------------------------------------------------------------------------------------
I
ab
=
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