Bookkeeping with Isotopic Patterns in Mass Spectra

10 Bookkeeping with isotopic Patterns in Mass Spectra 

Proving the identity of isotopic patterns requires careful comparison with calculated patterns. The mass differences must be consistent with the mass of the presumed neutral losses. In order to hold true, a pattern can only be assigned to signals at or above the mass given by the sum of all contributing atoms. 

Example The El mass spectrum of tribromomethane is dominated by the bromine isotopic distribution (Fig. 3.10). At first, there is no need to wonder why bromoform fragments like it does upon El (Chap. 6.1.4). Let us simply accept this fragmentation and focus on the isotopic patterns. By referring to Fig. 3.3 one can identify the patterns of Brs, Br2, and Br. As a matter of fact, the molecular ion must contain the full number of bromine atoms (jn/z 250, 252,254, 256). 

The primary fragment ion due to Br loss will then show a Br2 pattern m/z 171, 173, 175). A mass difference of 79 u is calculated between m/z 250 and m/z 171, owing to r, and thus identifying the process as a loss of Br . Starting from the CH Br2 Br isotopic ion at m/z 252 would yield the same information if Br was used for the calculation. Here, r would wrongly indicate loss of H2Br. 

Subsequent elimination of HBr leads to CBr at m/z 91, 93. Alternatively, the molecular ion can eliminate Br2 to form CHBr, m/z 92, 94, overlapping with the m/z 91, 93 doublet, or it may lose CHBr to yield Br2, causing the peaks at m/z 

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