Practical Aspects of Isotopic Patterns

The recognition of isotopic patterns bears some potential pitfalls. Particularly, if signals from compounds differing by two or four hydrogens are superirtposed or if such a superimposition can not a priori be excluded, the observed pattern has to be stepwise careMly checked to avoid misinterpretation of mass spectral data When isotopically labeled corrpounds are involved similar care also becomes necessary. 

The peaks in the m/z 50-57 range of the 1-butene El spectrum could be misinterpreted as a complex isotopic pattern if no formula were available on the plot (Fig. 3.8). However, there is no element having a comparable isotopic pattern and in addition, all elements exhibiting broad isotopic distributions have much higher mass. Instead, the 1-butene molecular ion undergoes H, H2 and multiple H2 losses. The m/z 57 peak, of course, results from In a similar fashion the peaks at m/z 39 and 41 appear to represent the isotopic distribution of iridium, but this is impossible due to the mass of iridium (cf. Appendix). However, these peaks originate from the formation of an allyl cation, CsHs, m/z 41, which fragments further by loss of H2 to form the CsHs ion, m/z 39 (Chap. 6.2.4). 

At this point, there is no need to worry about why bromoform behaves like this upon electron ionization. Instead it is sufficient to accept the occurrence of such fragments and to focus on the consequences in the appearance of the mass spectrum (Chap. 6.1.4). 

Note Proof of the identity of isotopic patterns requires careful comparison with those theoretically expected. Furthermore, the mass differences must be consistent with the mass of the presumed neutral loss. In no case can the pattern be observed in signals corresponding to lower mass than given by the sum of all isotopes included. 

Example From the tribromomethane spectrum (Fig. 3.9) a mass difference of 79 u is calculated between m/z 250 and m/z 171, which belongs to Br, thus identifying the process as a loss of a bromine radical. Starting from the CH r2 Br isotopic ion at m/z 252 would yield the same information if Br was used for the calculation. Use of Br would be misleading and suggest the loss of H2Br. 

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