Inficon Transpector XPR 3+ Operating Manual page 64

Transpector XPR 3+ Operating Manual
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 in nature have a mass of 14 AMU; only
0.37% have a mass of 15 AMU. Carefully examine the nitrogen spectrum in Figure
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4-2 on page 4-6. The largest peak at 28 AMU is the parent ion, N . The peak at
2
14 15 +
29 AMU is the isotope peak, N 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 (e.g., 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, are a very useful 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. That is, the probability of ionizing, for
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example, the mass 35 isotope of chlorine ( Cl) is the same as the probability of
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ionizing the mass 37 isotope ( Cl). Thus, the peak height ratio of mass 35 to 37
from HCl will be 3.07 to 1 (75.4% / 24.6%).
For a listing of the isotopic ratios for the lighter elements, see Table 4-2. 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.
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