Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 319

Figure 12.7 Mass spectrum of methanol (SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/
(National Institute of Advanced Industrial Science and Technology, accessed Aug. 2006)

ions at m/z of 31 and 32, the ions of 15 and 29 can be assigned to CH 3 and CHO þ ,
respectively, according to the following fragmentation patterns:

CH 3 OHþ1 electron ! CH 3 OH þ ðm=z 32Þþ2 electrons
CH 3 OH þ ðmolecular ionÞ!CH 2 OH þ ðm=z 31ÞþH
CH 3 OH þ ðmolecular ionÞ!CH 3 ðm=z 15ÞþOH
CH 2 OH þ ! CHO þ ðm=z 29ÞþH 2

Referring to the above mass spectrum of methanol, there are some small peaks (e.g., m/z at 30
and 33) which are less obvious with respect to their origins. Many of such ions are the results
of isotopes of the same element (C and H for methanol). This is better illustrated in the
following example.

EXAMPLE 12.2. Interpret the mass spectrum of 1,4-dichlobenzene (Fig. 12.8).

SOLUTION: The chemical formula of 1,4-dichlobenze is C 6 H 4 Cl 2 . Its exact molecular
weight depends on the isotopic composition of chlorine atoms (i.e., 35 Cl and 37 Cl). That is,
146 for C 6 H 3 4 Cl 2 , 148 for C 6 H 3 4 Cl 37 Cl, and 150 for C 6 H 3 4 Cl 2 . Note that the natural
abundance of 35 Cl and 37 Cl are 75.53% and 24.47%, respectively. In other words, these two
chlorine atoms have an approximate isotopic ratio of 35 Cl / 37 Cl ¼3:1. Such information can
be critical to the interpretation of mass spectra, particularly, when atoms such as chlorine and
bromine ( 79 Br: 81 Br¼50.69%: 49.31% 1:1) are present in the organic compounds.
The mass spectrum shows a base peak at m/z of 146, which corresponds to two
35 Cl atoms in this molecule (the most abundant isotopic form of 1,4-dichlor-
obenzene). Two other prominent molecular ion peaks (m/z¼148 and 150), although