SRS Labs QMS 100 Series, Руководство пользователя

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1–6 What’s Inside ?
SRS QMS Gas Analyzer
Mass Spectrometer
A residual gas analyzer (RGA) is mass spectrometer of small physical dimensions whose function is to
analyze the gases inside the vacuum chamber. The principle of operation is the same for all instruments:
A small fraction of the gas molecules are ionized to create positive ions, and the resulting ions are
separated, detected and measured according to their molecular masses. RGA’s are widely used to
quickly identify the molecules present in a gas, and when properly calibrated, can be used to determine
the concentrations or partial pressures of the components of a gas mixture.
The SRS RGA is a mass spectrometer consisting of a quadrupole probe, and an electronics control unit
(ECU) which mounts directly on the probe’s flange, and contains all the electronics necessary to
operate the probe. The computer software communicates only with the spectrometer. The RGA has
no direct knowledge that it is present in a QMS system; it simply analyzes what is present at the ionizer.
Figure 1-4. Quadrupole Head Components
The probe consists of three parts: the ionizer (electron impact), quadrupole mass filter and ion detector.
All of these parts reside in the vacuum space where the gas analysis measurements are made. The
ionizer converts the sample gas into an ionized gas that is collected by electric fields into the filter. The
quadrupole mass filter contains a precisely controlled high frequency electric field that allows only a
narrow mass range to pass through. The ions that pass through the filter are measured by the detector,
which is either the standard Faraday cup, or the optional electron multiplier detector.
The ECU is a densely packed box of electronics that attaches directly to the probe’s feedthru-flange. It
includes several regulated power supplies, a microprocessor, control firmware, and a standard RS232
communications port. The ECU contains a unique, temperature-compensated, logarithmic pico-
ammeter that can measure currents from 10
-15
to 10
-7
A. This large dynamic range exceeds the physical
limitations of RGA’s, which can achieve six orders of magnitude dynamic range during partial pressure
measurements. The high gain of the electron multiplier allows partial pressures better than 10
-13
mbar to
be detected. RGA’s operate in the “constant resolution” or “constant
∆
m” mode, with
∆
M
10%
preset
to one amu at the factory. The operating pressure range is from UHV to 10
-4
mbar. The RGA ionizer
will shut down if the total pressure rises above this limit.
Intelligent firmware, built into the RGA Head, completely controls the operation of the instrument, and
provides three basic modes of operation of the mass spectrometer:
•
Analog scanning