INFICON HAPSITE ER Chemical Identification System - Manual (Page 36)

1 - 8
IP
N 07
4-
47
1-
P1
B
HAPSITE ER Operating Manual
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. (A collision
would modify their trajectory, and possibly 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. At this point, 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 the software 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. The accumulation would raise the mass spectrometer pressure and
interfere 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. An additional diaphragm roughing
pump is provided.
The diaphragm pump consists of four stages, in series. The diaphragm pump
draws the gas from the exhaust of the compound pump and compresses the
exhaust gas sufficiently to discharge the exhaust into the atmosphere.