Instant Notes: Analytical Chemistry

E14 – Mass spectrometry 281

of about one-third the intensity atm/z51, the expected ratio for a species with

only one chlorine atom.

For 1-bromohexane, the almost equal abundances of the^79 Br and^81 Br isotopes

result in two pairs of nearly equal sized peaks at m/z164 and 166 (M and M+2)

and at m/z135 and 137 from the CH 2 CH 2 CH 2 CH 2 Br+fragment ion (Fig. 15). Both

the base peak at m/z43, due to the CH 3 CH 2 CH 2 +fragment ion, and that at

m/z85, due to the intact alkyl chain fragment ion, have no bromines to give

associated isotope peaks two mass units higher.

100

80

60

40

20

0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105

Relative abundance

CH 3 CH 2 CH 2

m/z

C

RMM = 102

O

OCH 3

M(102)

43

59

71

74

Fig. 12. Mass spectrum of methyl butyrate.

100

80

60

40

20

0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Relative abundance

m/z

M(74)

28

31

56

HO CH 2 CH 2 CH 2 CH 3

RMM = 74

Fig. 11. Mass spectrum of 1-butanol.