Inficon HAPSITE ER - Vacuum System

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HAPSITE ER Operating Manual
When the ions emerge from the mass selector, the ions are directed to the detector.
The active element of the detector is an electron multiplier. The electron multiplier
responds to the arrival of each individual ion with a cascade of electrons, each of
which generates more electrons. The result is a small burst of electrical current in
response to each ion emerging from the mass selector. The signal from the
electron multiplier is connected to the electronic amplifier and data-handling
system outside the vacuum.
In order to determine the constituents of the gas mixture, the ratio of RF to DC field
strengths is varied (swept) to permit progressively heavier ions to transit the mass
selector. The sweep, or scan, over the full range of masses (from 1 to 300 amu)
only takes about 100 milliseconds; the scan is usually repeated multiple times to
statistically improve the quality of the data. This scanning produces the mass
spectrum, a plot of the partial pressure or intensity of each mass.
The mass spectrum of the unknown compound is compared to a library of mass
spectra. The HAPSITE ER identifies the unknown compound based upon this
comparison.
2.4.3 Vacuum System
The Mass Spectrometer is operated in a vacuum for several reasons.
 The ions must travel 0.3048 m (12 in) 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 sample gas 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 created by the turbo-molecular and diaphragm pumps in the
Service Module. When a good vacuum is achieved, the pumps in HAPSITE ER are
turned on and the vacuum interconnect valve is closed. At this point, the Service
Module can be disconnected.
The two vacuum pumps of HAPSITE ER continue to provide the pumping
necessary for operation. These two pumps are the non-evaporable getter (NEG)
pump and the smaller sputter-ion pump. The NEG pump incorporates a special
zirconium alloy, arranged in sintered disks, which aggressively adsorbs gas
molecules when heated.
Over time, the sintered disks gradually become saturated with gas molecules,
which causes the adsorption ability to drop. The instrument detects the resulting
rise in operating pressure (loss of vacuum) and the software signals that the pump
must be replaced.