5–33
Transpector CPM Operating Manual
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, ESD/EID 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
ESD/EID.
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 (that is, the sum of ions of all masses) is electron energy and ionizer
dependent, a reasonable estimate for the number of ions produced (relative to a
standard, usually nitrogen) in a partial pressure analyzer is the relative ionization
gauge sensitivity.
5.3.3.2 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)
which decreases as mass increases.
The way the mass scale is calibrated (tuned)—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.
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