4 - 3
Transpector MPH 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 in 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, Transpector MPH has difficulty distinguishing 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 4-1; 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. This means
that an ion such as Ar
3+
will appear at 13.33 AMU, while F
2+
will show up
at 9.5 AMU.
4.1.1.2 Isotope Ratios
An additional cause of multiple peaks in the mass spectrum of a pure substance is
that most (but not all) elements are composed of more than one isotope. For
example, 99.63% of all nitrogen atoms have a mass of 14 AMU; only 0.37% have
a mass of 15 AMU. Examine the nitrogen spectrum in Figure 4-1 on page 4-2. The
largest peak at 28 AMU is the parent ion, N
2
+
. The peak at 29 AMU is the isotope
peak,
14
N
15
N
+
, and is 0.74% (two times 0.37%) as high as the parent peak since
there are two nitrogen atoms in the ion, each one of which has a 0.37% chance of
being 15 AMU.
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