Several elements of interest to organic chemists have modest to high abundance heavy isotopes. These are silicon, sulfur, chlorine and bromine as shown in Table 1. Heavy isotopes of chlorine and bromine are especially noteworthy because they are so abundant. 37Cl is approximately one third of the abundance of 35Cl and 81Br is approximately equal to the abundance of 79Br. As a result, the relative intensities of the molecular ion peak and its associated isotope peaks directly indicate the number of chlorine and bromine atoms in the molecular formula. For example, a molecule with one bromine will show a peak at m/z=M and a peak of almost equal intensity at m/z=M+2. A molecule with two bromines will show peaks at m/z=M, M+2 and M+4 with the approximate intensity ratio of 1:2:1. Intensity patterns characteristic of the presence of various numbers of chlorines and bromines in a molecular formula are shown in Figure 1. Of course, superimposed on the effect of the isotopes of chlorine and bromine are the smaller effects of the isotopes of silicon, sulfur, carbon, hydrogen, nitrogen, and oxygen if present as given in Table 1.
Figure 1. Relative intensities of the molecular ion and its major isotope peaks when various numbers of chlorines and bromines are present in the molecular formula. The diagrams do not include the effect of other isotopes which will be present such as 13C.
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