Organic Chemistry

488 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

13.3 Isotopes in Mass Spectrometry

Although the molecular ions of pentane and 2-methylbutane both have mzvalues of

72, each spectrum shows a very small peak at (Figures 13.2 and 13.3). This

peak is called an peak because the ion responsible for it is one unit heavier

than the molecular ion. The peak owes its presence to the fact that there are

two naturally occurring isotopes of carbon: 98.89% of natural carbon is and

1.11% is (Section 1.1). So 1.11% of the molecular ions contain a instead of a

and therefore appear at

Peaks that are attributable to isotopes can help identify the compound responsible

for a mass spectrum. For example, if a compound contains five carbon atoms, the rel-

ative intensity of the ion should be multiplied by the

relative intensity of the molecular ion. This means that the number of carbon atoms

in a compound can be calculated if the relative intensities of both the M and

peaks are known.

The isotopic distributions of several elements commonly found in organic com-

pounds are shown in Table 13.2. From the isotopic distributions, we see why the

peak can be used to determine the number of carbon atoms in a compound: It

is because the contributions to the peak by isotopes of H, O, and the halogens

are very small or nonexistent. This formula does not work as well in predicting the

number of carbon atoms in a nitrogen-containing compound because the natural abun-

dance of^15 Nis relatively high.

M+ 1

M+ 1

number of carbon atoms=

relative intensity of M+ 1 peak

.011* 1 relative intensity of M peak 2

M+ 1

M+ 1 51 1.1% 2 = 51 .011 2 ,

(^12) C M+1.
(^13) C (^13) C
(^12) C
M+ 1
M+ 1
m>z= 73

Element Natural abundance
Carbon
98.89% 1.11%
Hydrogen
99.99% 0.01%
Nitrogen
99.64% 0.36%
Oxygen
99.76% 0.04% 0.20%
Sulfur
95.0% 0.76% 4.22% 0.02%
Fluorine
100%
Chlorine
75.77% 24.23%
Bromine
50.69% 49.31%
Iodine
100%
(^127) I
(^79) Br (^81) Br
(^35) Cl (^37) Cl
(^19) F
(^32) S (^33) S (^34) S (^36) S
(^16) O (^17) O (^18) O
(^14) N (^15) N
(^1) H (^2) H
(^12) C (^13) C
Table 13.2 The Natural Abundance of Isotopes Commonly Found
in Organic Compounds