Instant Notes: Analytical Chemistry

280 Section E – Spectrometric techniques

100

80

60

40

20

0

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

Relative abundance

CH 3

RMM = 92

m/z

91

M(92)

Fig. 9. Mass spectrum of methylbenzene.

100

80

60

40

20

0

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

Relative abundance

CH 3 C

O

RMM = 72

m/z

43

57

M(72)

CH 2 CH 3

Fig. 10. Mass spectrum of butanone.

peak at m/z43 arises from the CH 3 CH 2 CH 2 +fragment ion. Peaks at m/z59 and 71

are from the CH 3 OCO+and the CH 3 CH 2 CH 2 CO+fragment ions respectively.

The spectrum of benzamide(Fig. 13) demonstrates the stability of an aromatic

ring structure (Rule 4). After the base peak at m/z 77, due to the C 6 H 5 +fragment

ion, the next two most intense peaks (m/z105 and 121) are from the C 6 H 5 CO+

fragment ion and the molecular ion, respectively. The former loses a neutral CO

molecule to give the base peak, and then a neutral C 2 H 2 molecule to give the

C 4 H 3 +fragment ion at m/z51.

In the spectrum of dichloromethane(Fig. 14), the relative intensities of the

m/z84 (M), 86 (M+2) and 88 (M+4) peaks, correspond to the pattern expected

from a molecule with two chlorine atoms whose isotopes are^35 Cl and^37 Cl

(Table 3). The base peak is at m/z49, from the CH 235 Cl+fragment ion after the loss

of one chlorine from the molecular ion. This is accompanied by a CH 237 Cl+ion