Inficon Transpector XPR 3+ - Page 77

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Transpector XPR 3+ Operating Manual
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 TS2 through 5, 16, 28, and 30 of Partial
Pressure Analyzers and Analysis by Drinkwine and Lichtman for more information
on EID/ESD.
Partial pressure analyzers are 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 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 a partial pressure analyzer 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
characteristic provides a good degree of peak separation throughout the mass
spectrum, but results in an ion transmission efficiency (i.e., the fraction of all ions
of the selected mass entering the mass filter which are transmitted through it) that
decreases as mass increases.
The way the mass scale is “calibrated” or “tuned” (i.e., the way the peak positions
and widths are adjusted) can have a significant effect on the transmission efficiency
of the mass filter across the mass spectrum. If the adjustments are not made
properly, the ratios of peak heights across the mass range will not be correct.