Inficon Transpector MPH - Page 77

4 - 12
Transpector MPH Operating Manual
The analyzer factor, A
b
, depends on the transmission and detection characteristics
of the analyzer, the Electron Multiplier gain (if the analyzer is so equipped), and the
basic sensitivity, as indicated in equation [4]:
[4]
Here, TF
b
is the transmission factor of the mass filter at mass b. The transmission
factor is the fraction of ions at mass b which pass through the mass filter, relative
to nitrogen ions at mass 28. Nominally, the transmission factor is equal to 28
divided by the mass of the ion, b.
The detection factor, DF
ab
, is equal to 1 for a Faraday Cup detector. For an Electron
Multiplier, the detection factor is a function of the mass of the ion and its chemical
nature, and is measured relative to that of a reference gas, typically nitrogen.
In general, as the mass ion increases, the Electron Multiplier detection factor
decreases.
The gain of the Electron Multiplier, G, measured at mass 28 for nitrogen, is the
Electron Multiplier output current divided by the Faraday mode output current,
under otherwise identical conditions. The multiplier gain is a strong function of the
high voltage applied.
The sensitivity of the instrument, S, is the ratio of Faraday mode ion current for a
given pressure of pure nitrogen measured at mass 28, and is typically expressed
in amps/Torr.
cyclohexane C
6
H
12
6.4 propane C
3
H
8
3.7
deuterium D
2
0.35 silver perchlorate AgClO
4
3.6
dichlorodifluormethane CCl
2
F
2
2.7 stannic iodide Snl
4
6.7
dichloromethane CH
2
Cl
2
7.8 sulfur dioxide SO
2
2.1
dintrobenzene C
6
H
4
(NO
2
)
2
7.8 sulfur hexafluoride SF
6
2.3
ethane C
2
H
6
2.6 toluene C
6
H
5
CH
3
6.8
ethanol C
2
H
5
OH 3.6 trinitrobenzene C
6
H
3
(NO
2
)
3
9.0
ethylene oxide (CH
2
)
2
O 2.5 water H
2
O1.0
helium He 0.14 xenon Xe 3.0
hexane C
6
H
14
6.6 xylene C
6
H
4
(CH
3
)
2
7.8
hydrogen H
2
0.44
Table 4-5 Ionization Probabilities for some Common Substances (continued)
Substance Formula
Relative
Ionization
Gauge
Sensitivity
Substance Formula
Relative
Ionization
Gauge
Sensitivity
A
a
1
TF
b
DF
ab
 G S
--------------------------------------------------=