Instant Notes: Analytical Chemistry

E14 – Mass spectrometry 279

The spectrum of methylbenzene(Fig. 9) typifies alkyl-substituted aromatic

compounds, with a base peak corresponding to the tropylium ion, C 7 H 7 +, at

m/z91 and a large molecular ion peak at m/z92 (Rules (iv) and (v)).

The base peak of butanone (Fig. 10) is due to the resonance-stabilized

CH 3 CO+fragment ion, m/z43, originating from cleavage of the C-C bond adja-

cent to the carbonyl group (Rule (vii)). The loss of a methyl radical gives a

prominent peak at m/z57 from the CH 3 CH 2 CO+fragment ion.

Alcohols readily lose water to give (M-18)+fragment ions (Rule (vi)). In the

case of 1-butanol(Fig. 11), this occurs at m/z56. The molecular ion peak is corre-

spondingly small, and the base peak at m/z31 is due to CH 2 OH+.

Methyl butyrateprovides a good example of a McLafferty rearrangement

peak in a carbonyl compound (Rule (vii)). In this spectrum (Fig. 12), the peak,

which is due to the CH 2 =CO+HOCH 3 fragment ion, is atm/z74, while the base

100

80

60

40

20

0

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115

Relative abundance

29

43

57

71

85

M(114)

CH 3 (CH 2 ) 6 CH 3

RMM = 114

m/z

Fig. 7. Mass spectrum of octane.

100

80

60

40

20

0

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115

Relative abundance

M(114)

CH 3 C

CH 3

CH 3

RMM = 114

m/z

CH 2

CH 3

CH CH 2

57

Fig. 8. Mass spectrum of 2,2,4-trimethylpentane.