4 - 13
Transpector MPS Operating Manual
The overall relation between partial pressure and ion current, given in
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]
A brief discussion of each of the terms follows:
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
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 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
=
