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Transpector MPS Operating Manual
Third, 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 (Chapter 8, Section 2) for more information on filament
materials and their interactions with the gas being analyzed. An example of gases
being produced from surfaces is the liberation of carbon monoxide molecules from
a Yttria oxide coated iridium filament by a sputtering mechanism in the presence of
significant quantities of argon. This latter mechanism makes the combination of a
pressure reduction system and a Transpector MPS sensor unsuitable for
measuring nitrogen contamination in argon at the low parts-per-million (PPM) level
from a sputter deposition process. A special type of inlet system and ion source
(often referred to as a Closed Ion Source (CIS)) should be used for this type of
application.
Fourth, 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 (such as
sulfur hexafluoride, chlorofluorocarbons, perfluorotributylamine, or
perfluorokerosene) for extended periods of time, it is not uncommon for a strong
F
+
peak at 19 AMU to remain even after the fluorine containing substance has been
removed. When operating in the UHV region, EID/ESD of H
+
, C
+
, O
+
, and CO
+
(and other ions) is not uncommon. The clue to diagnosing this problem is that the
observed fragmentation patterns do not match known gas phase patterns. See
pages five and six, and typical spectra TS-2 through 5, 16, 28, and 30 of Partial
Pressure Analyzers and Analysis by Drinkwine and Lichtman for more information
on EID/ESD.
Transpector MPS is also characterized by varying degrees of mass discrimination;
that is, the sensitivity of the instrument is a function of mass. Ion sources show
mass discrimination because various substances offer different degrees of difficulty
of ionization. Generally, heavy, large molecules are ionized more readily than light,
small molecules. There is also a rough correlation between the number of electrons
in a molecule and its ease of ionization. Although the total ion yield (i.e., the sum
of ions of all masses) is electron energy and ionizer dependent, a reasonable
estimate for the number of ions produced (relative to some standard, usually
nitrogen) in an Transpector MPS is the relative ionization gauge sensitivity.
4.1.3.2 Scanning Characteristics
Quadrupole mass filters can also exhibit mass discrimination characteristics
depending on how the control voltages are varied during the sweep through the
mass range. Most instruments are designed to operate with a constant peak width
(constant
M) which results in a resolution which is proportional to the mass. This
