Thomas J. Bruno and Paris D. N. Svoronos
Because of the distinctive mass spectral patterns caused by the presence of chlorine and bromine in a molecule, interpretation of some mass spectra can be much easier as the relative isotopic concentrations are known. The following table provides ideal peak intensities, relative to the molecular ion (M+) at an intensity normalized to 100%, for various combinations of chlorine and bromine atoms, assuming the absence of all other elements except carbon, hydrogen, and oxygen (Refs. 1-4). Note that in the table we refer to the base peak (the peak with the highest intensity) as P, as it might not always be the molecular ion. The mass abundance calculations were based upon the most recent atomic mass data (see the table “Atomic Masses and Isotopic Abundances” in Section 1 of this CRC Handbook).
Note that experimental uncertainty can change these ideal projections; many users will have an instrument capable of only unit mass resolution. In such cases, one must expect these patterns to be approximate. For example, if a molecule has no Cl and two Br, the P/(P+2)/(P+4) ratio is roughly 100/200/100 (P is assumed to be 100, and is not listed in the table). To confirm the presence of two bromine atoms, look for a peak at P-79, attributed to the loss of one bromine atom. Losing one bromine would lead to a “doublet” (roughly 1:1 ratio) which is indicative of a fragment containing only one bromine atom. Furthermore, at P-158 one would see that the 1:1 pattern has disappeared. These two observations will confirm the presence of two bromine atoms in the original P+ base peak ion.
If a molecule has two Cl atoms and no Br atoms present, the P/(P+2)/(P+4) ratio is roughly 100/67/11 in a similar fashion. To confirm, one should consider the P-35 area of the spectrum (loss of one chlorine). Losing one chlorine would lead to a doublet (roughly 3:1 ratio). In general, no lost fragments of mass 35 or 79 are indicative of other atoms or fragments. Occasionally a halogen is lost as the hydrogen halide which will lead to the loss of mass 80 (HBr) or mass 36 (HCl), respectively, but the doublet pattern P/P+2 remains the same.
The spectrum becomes much more complicated when more than two bromine, or two chlorine atoms, or a combination of both atoms, is involved, as one can observe in the table. The rule of thumb, however, is that one ought to always look at the peaks (P-79) or (P-35) to alert the presence of a bromine or chlorine, respectively. We also note that the statistical likelihood of encountering molecules with high multiples of Cl and Br atoms is low outside of environmental analyses.
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