INFICON HAPSITE Smart Plus Chemical Identification System - Manual (Page 36)

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HAPSITE Smart Plus Operating Manual
the rods. Once on the rod, the heavier ion is neutralized. At a specific combination
of RF and DC fields, ions of a specific mass will be able to transit the rod structure
and emerge at the exit end to be detected.
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 sweep is usually repeated many times to
statistically improve the quality of the data. This produces the mass spectrum, a
plot of the partial pressure (or population or intensity or amplitude) of each mass.
The mass spectrum is compared with a library of mass spectra characteristic of
many individual compounds, and the HAPSITE reports the compounds which
match the observed spectrum.
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. (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.