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Transpector SPS Operating Manual
5.3.3 Additional Information For Interpreting Mass Spectra
5.3.3.1 Ion Source Characteristics
A closed ion source, and the particular inlet system selected, can have an effect on
the mass spectrum obtained.
The analyzer itself is a source of gas molecules because of outgassing from its
surfaces. Usually, 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). Overnight bakeouts at the maximum
allowable temperature is the best way to minimize the effects of outgassing of the
sensor, manifold, and inlet system.
NOTE:
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
down.
CAUTION
Make sure EM high voltage is off if the bakeout
temperature exceeds 150°C. Otherwise, permanent
damage to the EM may result.
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
. 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 the ion source.
Reactions involving gas molecules on surfaces of the analyzer can result in a
change of composition. Gases can either be consumed by the surfaces, or
produced by the surfaces. One example of gas consumption is the reaction of
oxygen with a hot filament, particularly when tungsten filaments are used. The
typical result is an anomalously low concentration of oxygen detected.
See O’Hanlon’s book for more information on filament materials and their
interactions with the gas being analyzed. (See
.) An
example of gases being produced from surfaces is the liberation of carbon
monoxide molecules from a thorium oxide coated iridium filament by a sputtering
mechanism in the presence of significant quantities of argon. It is for this reason
that the PVD version of the CIS uses tungsten filaments.
There are cases where at least some of the ions detected are emitted from
surfaces in the ion source under electron bombardment and are not generated in
the gas phase from neutral molecules. This process is known as electron
stimulated desorption (ESD), or sometimes as electron induced desorption (EID).
When the sensor has been exposed to fluorine containing substances (e.g., sulfur
hexafluoride, chlorofluorocarbons, perfluorotributylamine, or perfluorokerosene)
for extended periods of time, it is not uncommon for a strong F
+
peak at 19 AMU to
