Inficon HAPSITE ER, Руководство по эксплуатации

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7 | Introduction INFICON
26 / 436 074-471-P1E HAPSITE ER Operating Manual
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.
7.8 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.
The NEG pump can effectively remove active gases, but not noble gases. An ion
pump is necessary to pump out noble gases, which would accumulate in the mass
spectrometer. The accumulation would raise the mass spectrometer pressure and
interfere with operation.
The turbomolecular 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.
However, even with drag stages, the turbo molecular pump is unable to exhaust gas
into atmospheric pressure. An additional diaphragm roughing pump is provided for this
purpose.