Inficon Transpector MPH - Page 78

4 - 13
Transpector MPH Operating Manual
The overall relation between partial pressure and ion current, given in equation [5],
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.
[5]
Where:
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 4-4 on page 4-10)
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
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 am ps; assume s 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
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I
ab
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