Inficon Transpector XPR 3+, Руководство по эксплуатации

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4 - 3
Transpector XPR 3+ Operating Manual
In all cases, the reactants are a high energy electron, e
-
, and a gas molecule, XYZ.
The products of the first reaction are the molecule with a single electron removed
(the so-called parent ion) and two low energy electrons. In the second reaction, two
electrons are removed from the gas molecule, resulting a doubly charged ion.
Triply (or even more highly) charged ions are also possible, provided the incident
electron has enough energy.
Reactions 3 through 8 are all examples where the original molecule is broken into
fragments, at least one of which is positively charged (negative ions can also be
produced in this manner). Only the positive ion fragments are observed; the neutral
(i.e., uncharged) fragments are not detected. The mass spectrum obtained when
the parent molecule breaks apart under electron impact is commonly referred to as
the fragmentation pattern (or, sometimes, the cracking pattern). For example, a
fragmentation pattern for Nitrogen shows
14
N
+
(14 AMU),
14
N
2
+
(28 AMU), and
14
N
15
N
+
(29 AMU).
In general, peaks from multiply charged species will be less intense than those for
the corresponding singly charged ion. For example, the doubly charged peak for
argon is typically less than one fifth as intense as the singly charged peak (it should
be noted that this intensity ratio is sensitive to the incident electron energy).
There are some situations when it is difficult to determine whether the ion is singly
or multiply charged. When a molecule is composed of two atoms of the same
element the typical partial pressure analyzer cannot distinguish between the singly
charged one-atom fragment ion and the doubly charged two-atom molecular ion,
which will both have the same mass-to-charge ratio. Refer to
Figure 4-2 on page
4-6 ; the peak at 28 AMU is the parent ion, N
2
+
. It is not discernible from this
spectrum if the peak at 14 AMU is from N
+
or N
2
2+
. It has been demonstrated, by
other means, that the 14 AMU peak in the nitrogen spectrum is from the singly
charged fragment ion.
Most ions (with the important exception of complex hydrocarbons) have masses
very close to integer values. When the mass of an ion is not evenly divisible by the
number of charges on it, the mass-to-charge ratio will not be an integer. Thus, Ar
3+
will appear at 13.33 AMU, while F
2+
will show up at 9.5 AMU.