SRS Labs QMS 100 Series, Руководство пользователя

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2-20 Ion Detector
SRS QMS Gas Analyzer
along the channel walls replenishing their charge as secondary electrons are emitted. Channel electron
multipliers, operate linearly in the analog mode until the output current is approximately 10%
of the bias current.
The dark current of a multiplier is the electron current measured at its output in the absence of an input
ion current. The minimum output current that can be accurately measured with the multiplier is
equal to the dark current noise.
Example: For a typical resistance of 200 M
Ω
, and a bias voltage of -2000V, the bias current is 10
µ
Amps and the output current must be kept under 1
µ
Amp. Since the gain at that voltage is roughly
10
6
, the maximum input current at which the output current behaves linearly is 10
-12
Amps (1
µ
Amp /
10
6
). Typical dark currents are lower than 10
-13
Amp, and the minimum input current that can be
detected is 10
-19
Amps. For a sensitivity of 10
-4
Amp/Torr, this corresponds to an lower and upper
limits of 10
-15
and 10
-8
Torr, respectively, and 7 orders of magnitude of dynamic range.
The total gain of electron multipliers varies as a function of the mass of the incident ions. As a rule of
thumb, and for small molecules,
the gain decreases as mass increases . This mass discrimination
effect is caused by the dependence of ion-electron conversion efficiencies on the velocities of the ions
entering the detector. For example, an inverse relationship with the square root of the mass has been
reported for monoatomic ions of the same energy. For accurate quantitative measurements, it is essential
to calibrate in advance the gain of the multiplier for the specific ionic species being detected.
An important problem when working with multipliers is that their gain changes with time. Gain
degradation is unavoidable, and particularly serious just after the detector has been exposed to air, or
after high quantities of reactive gases have been introduced into the vacuum system. The increased
surface area provided by the extra channels in the multi-channel devices reduces this problem; however,
frequent calibration of the multiplier gain against the FC output is recommended for reliable quantitative
measurements. This is done automatically with the RGA Windows

software.
Gain degradation limits the lifetime of all electron multipliers. Eventually the gain drops to unacceptable
values and the multiplier needs to be replaced. As a rule of thumb, the multiplier should be replaced
when the required gains can no longer be achieved by increasing the bias voltage. The lifetime
of electron multipliers is ultimately dependent upon the accumulated charge drawn from the multiplier
(
Gain degradation typically starts at accumulated output current values of a few thousand
µµ
Amp-hr) . However, the lifetime also depends critically on the residual gas environment and the
duration of transient signals. Contamination by organic compounds (i.e. diffusion or mechanical pump
oil) and the interaction with highly reactive gases must be avoided at all times.
It has been found that, in many cases, channel multipliers may be successfully refreshed by cleaning
them in high purity isopropyl alcohol. The procedure is described in the RGA Maintenance chapter
(CDEM Refreshment section) and, even though
it is not guaranteed to always work , it is worth
trying as a last resort before discarding a multiplier.
Channel electron multipliers have a history of high performance and dependability in mass spectrometry