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Transpector MPS Operating Manual
4.1.3 Additional Information for Interpreting Mass Spectra
4.1.3.1 Ion Source Characteristics
It is important to recognize that Transpector MPS (especially the ion source) and
the vacuum system configuration can both have an effect on the relative
concentrations of the gases detected. In order to minimize these effects, have the
right type of ionizer, the right type of filament, and the right configuration of the
vacuum system. This is particularly true when a differential pumping arrangement
is used because the pressure of the gas to be sampled is too high for the sensor
to operate. J. O’Hanlon’s book,
A User’s Guide to Vacuum Technology
, has a brief
discussion (in Chapter 8, Section 2) of some of these concerns.
When using Transpector MPS as a residual gas analyzer, the sensor should be
installed such that the conductance between the ion source and the vacuum region
to be analyzed is maximized. If possible, install the sensor without any intervening
valves or vacuum hardware. If the sensor is equipped with a residual gas analyzer
ion source, there are four classes of interactions between the sensor and the
immediate vacuum environment which can have a significant effect on the detected
gas composition.
First, the analyzer itself is a source of gas molecules because of outgassing from
its surfaces. Usually, the outgassing levels can be reduced by baking the analyzer
in vacuum and by using the Degas function (wherein the ion source surfaces are
bombarded by high energy electrons). When operating in the ultra-high vacuum
(UHV) region, it is best to bake the sensor overnight at the maximum permissible
temperature with the electronics removed and thermal insulation applied around
the sensor feedthrough flange. A second overnight bakeout should be performed
at the maximum sensor operating temperature. (It can take more than three hours
for all parts of the sensor to reach maximum temperature during a bakeout, and
more than six hours to cool back down.)
CAUTION
Ensure that the Electron Multiplier is turned off if the
(second) bakeout temperature exceeds the maximum
EM operating temperature. Otherwise, permanent
damage to the EM may result.
Second, it is possible that the opposite of outgassing can occur; that is, gas
molecules can be captured by the surfaces of the sensor. This effect is called
pumping. In such cases, the magnitude of the signals of the gases pumped will be
lower than is properly representative of the composition of the gas in the vacuum
chamber. Significant temporary pumping effects will frequently occur following
degassing of the ion source.
