Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 104
Vacuum Technology
Figure 4.17: Operating principle of a C-SEM
4.1.2.4 Vacuum system
Pressures of less than 10 - 4 mbar are required for operation of quadrupole mass spectrometers.
This necessitates an appropriate pumping station with pressure monitoring. In order to perform
gas analysis with optimal sensitivity, only a low a base pressure is necessary and the
residual gas should only contain unavoidable partial pressures stemming from desorption from
the walls of the equipment. Residual gas spectra of this type and low base pressures are best
attained with turbo drag pumping stations (Figure 2.22). An additional total pressure gauge
protects the mass spectrometer against being energized at excessively high pressures.
When setting up such a system, attention must be paid to meaningful arrangement of gas
inlet, valves, pumps and measurement instruments in order to avoid falsification stemming
from unfavorable flow conditions. A separate pumping station that evacuates the measurement
system is often required during the course of vacuum processes that run at high
pressure. Small pumping stations with turbo drag pumps and diaphragm pumps are used
for this purpose.
4.1.2.5 Inlet system
Many vacuum technology processes that are monitored by mass spectrometers occur in
pressure ranges of more than 10 - 4 mbar. Gases to be analyzed must also be relieved from
atmospheric pressure to pressures of less than 10 - 4 mbar. Differing pressure reducing procedures
are used, depending on the pressure gradient in question.
Gas mixtures should be admitted to the mass spectrometer without de-mixing if possible:
At pressures p of more than 10 mbar, pressure is reduced by means of a (heatable)
capillary, in which laminar flow prevails, with a downstream gas inlet valve. Under some
circumstances, pressure reduction by means of an additional pump will be necessary
upstream of the valve
U MESS
Ion +
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Conversion dynode
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