Inficon HAPSITE Smart, Operating Manual

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HAPSITE Smart Operating Manual
Alternatively, the mass spectrometer can remain tuned to a specific mass or set of
masses. The instrument measures the partial pressure of only those masses as a
function of time. Operation in this mode, termed selected ion monitoring (SIM),
permits very sensitive measurement of the presence of one or a few compounds
which have already been identified.
1.6.3 Vacuum System
The mass spectrometer is operated in a vacuum for several reasons.
 The ions must travel nearly a foot from the ionizer through the quadrupole to
the electron multiplier without colliding with another molecule (because this
would modify their trajectory, and maybe their charge).
 The gas to be analyzed must be free from interference from other unknown
gases.
 The hot filament which generates the electrons would be destroyed if operated
at atmospheric pressure in the presence of oxygen.
The vacuum is initially provided by the turbo-molecular and diaphragm pumps in
the Service Module. When a good vacuum level is achieved and the pumps in the
HAPSITE are turned on, the vacuum interconnect valve is closed and the Service
Module can be disconnected. The Service Module is not needed again until the
NEG pump in the HAPSITE must be changed.
The two vacuum pumps of the HAPSITE continue to provide the pumping
necessary for operation. These two pumps are the non-evaporate getter (
NEG )
pump and the smaller sputter-ion pump. The NEG pump incorporates a special
zirconium alloy, arranged in sintered disks, which when heated adsorb gas
molecules very aggressively.
Over time, the sintered disks gradually become fully saturated with gas molecules
and the pumping speed drops. The instrument detects the resultant rise in
operating pressure (loss of vacuum) and signals that the pump must be replaced.
The NEG pump is very effective in removing the active gases, but the NEG pump
does not remove noble gases. The sputter-ion pump is provided to remove argon,
neon, helium, krypton, and xenon which would otherwise accumulate in the mass
spectrometer, raising the mass spectrometer pressure and interfering with
operation.
The turbo molecular pump in the Service Module is actually a compound pump,
incorporating turbo molecular stages for high pumping speeds at low pressure, and
molecular drag stages to provide good compression of the gas at higher pressures.
Even with the drag stages, this pump is unable to compress the gas enough to
exhaust the gas into atmospheric pressure, so an additional diaphragm roughing
pump is provided.