5–15
Transpector CPM Operating Manual
5.3.1.2 Isotope Ratios
An additional cause of multiple peaks in the mass spectrum of a pure substance is
that most elements are comprised of more than one isotope. For example, 99.63%
of all nitrogen atoms in nature have a mass of 14 AMU; only 0.37% have a mass
of 15 AMU. Examine the nitrogen spectrum in Figure 5-7. 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.
Some elements have many intense isotopes. For example, xenon is
0.096% mass 124, 0.090% mass 126, 1.92% mass 128, 26.44% mass 129,
4.08% mass 130, 21.18% mass 131, 26.89% mass 132, 10.44% mass 134,
and 8.87% mass 136.
Isotope ratios, like fragmentation patterns, aid in recognizing specific materials.
Under normal partial pressure analyzer ionization conditions, the peak height ratios
for the various isotopes of an element will be the same as the ratios of their natural
abundance’s. For example, the probability of ionizing the mass 35 isotope of
chlorine (
35
Cl) is the same as the probability of ionizing the mass 37 isotope (
37
Cl).
Thus, the peak height ratio of mass 35 to 37 from HCl will be 3.07 to 1
(75.4% / 24.6%).
Table 5-2 lists the isotopic ratios for lighter elements. For a complete listing of the
natural abundances for the isotopes of all the elements, see the Handbook of
Chemistry and Physics from CRC Press.
Table 5-2 Isotope ratios
Isotope Ratios
Element Mass No. Relative
Abundance
H 1 99.985
20.015
He 3 0.00013
4 ~100.0
B 10 19.78
11 80.22
C 12 98.892
13 1.108
N 14 99.63
15 0.37
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