Compositions of All Peaks Where Possible

For the pieces of the molecule displayed in its mass spectrum, knowledge of the elemental composition, as well as the mass, is also very valuable in structural elucidation. Even without exact mass measurement, one can often restrict the possibilities for elemental-composition assignment for important peaks by using isotopic-abundance data. Here the chief difficulty is that the abundance measured at a particular mass value can actually represent contributions from more than one elemental composition. Such interferences are possible even for isotopic peaks of molecular ions. 

Maximum number of atoms of the element. In calculating the elemental composition of a peak A above, we assumed that the only contributions to the (A + 1) and (A + 2) peaks were from the less-abundant isotopes of the elements in A. However, this is not necessarily true there could be important additional contributions to the (A + 1) and (A + 2) peaks (and, occasionally, to peak A) for which corrections must be made. For the (A + 1) and (A + 2) peaks, these could 

Interference from other figment ions. In Unknown 2.4 calculating the number of carbon atoms of m/z 41 from the relative abundance of ions in i/z 42 gives a ridiculously high number of carbon atoms [42 ]/[4U] = 12/27 = 44%, corresponding to 40 carbon atoms in Table 2.2. Although this is correct for the maximum number of C atoms, it is hardly a helpful calculation the reason for this value is that most of the m/z 42 peak arises from another frag- 

Thus the most useful isotopic-abundance calculations are generally those that use the significant peaks which are at the high-mass end of peak groups. Can you deduce the elemental compositions of any other ions in Unknowns 2.5 and 2.6  

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