Instant Notes: Analytical Chemistry

276 Section E – Spectrometric techniques

CNH 2 C 6 H 5

–C 2 H 2

NH 2 C 6 H 5 C

m/z 105

m/z 51 m/z 77

m/z 44

M•+ m/z 121

+

+e– –2e C O

O O

–NH 2 •

–C 6 H 5 •

CNH 2

O +

C 4 H 3 + + = C 6 H 5 +

–CO

Fig. 6. Fragmentation pathways for EI ionization of benzamide (spectrum shown in Fig. 13).

percentage relative abundances. Thus, natural carbon consists of a mixture of

98.90%^12 C and 1.10%^13 C, and natural hydrogen is a mixture of 99.985%^1 H and

0.015%^2 H (deuterium).

The very small peaks above the molecular ion peak, M+, at m/z32 in the spec-

trum of methanol (Fig. 1) are due to the heavier isotopes of carbon, hydrogen

and oxygen. These isotope peaks, designated (M+1)+, (M+2)+etc., are of impor-

tance in the interpretation of mass spectra and can be used for two purposes.

(i) To establish an empirical formula for molecules containing C, H, O and N

by comparison of the relative intensities of the M, M+1 and M+2 peaks in a

recorded spectrum with tabulated values. Table 2is an extract of extensive

Table 1. Natural isotopic abundances of some common elements as a percentage of the

most abundant isotope

Element Most abundant Other isotopes Abundance of

isotope heavier isotopea

Hydrogen^1 H^2 H 0.016

Carbon^12 C^13 C 1.11

Nitrogen^14 N^15 N 0.38

Oxygen^16 O^17 O 0.04

(^18) O 0.20
Sulfur^32 S^33 S 0.78
(^34) S 4.40
Fluorine^19 F 100
Chlorine^35 Cl^37 Cl 32.5
Bromine^79 Br^81 Br 98.0
Iodine^127 I 100
Phosphorus^31 P 100
aRelative to 100 for the most abundant.