Multiple Isotopic Compositions at Very High Resolution

1 Multiple Isotopic Compositions at Very High Resolution 

During the introduction of resolution it should have become clear that very high resolution is capable of separating different isotopic compositions of the same nominal mass, thereby giving rise to multiple peaks on the same nominal m/z (Figs. 3.18, 3.19, 3.22). In molecules with masses of around 10,000 u, a single unresolved isotopic peak may consist of as many as 20 different isotopic compositions. [38] 

Example Peptides often contain sulfur from cysteine. Provided there are at least two cysteines in the peptide molecule, the sulfur can be incorporated as thiol group (SH, reduced) or sulfur bridge (S-S, oxidized). Often, both forms are contained in the same sample. At ultrahigh-resolution, the contributions of these compositions to the same nominal m/z can be distinguished. The ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) FT-ICR mass spectrum of native and reduced [D-Pen jenkephalin gives an example of such a separation (Fig. 3.25). [39] The left expanded view shows fully resolved peaks due to and C2 isotopomers of the native and the all- C peak of the reduced compound at m/z 648. The right expansion reveals the peak of the native plus the 

We started our lesson on isotopic pattams with the unspoken assunption of nominal mass resolution, which makes it much easier to understand how isotopic patterns are formed. The procedure of summing up all isotopic abundances contributing to the same nominal mass is correct as long as very high resolution is not employed. In that case, following such a simplified protocol is acceptable, because generally isobaric isotopolog ions are very similar in mass [24]. 

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