Figure 5.

Identification of S‐containing fragments using exact mass data: appearance of m + 2 isotopologues at +1.995796 with a precision of 0.05‰ in a standard mixture containing methionine and cysteine (a) and an extract from Arabidopsis leaves (b). Ions that effectively contain S are circled in red. Other ions do not contain S despite their mass difference value (w.r.t. the isotopologue at +1.995796) close to zero (see main text for calculations). In (a) and (b), red frames illustrate the chemical structure of identified S‐containing fragments. (c) detailed observed isotopic pattern (m + 2 isotopologues) of the fragment ion with a monoisotopic mass of 74.018 Da, showing the ³⁴S isotopologue (observed at +1.99549) of structure 2 in (b), as well as ³⁰Si and ²⁹S + ¹³C isotopologues of C₂H₆OSi (74.018791 Da), the monoisotopic form of which also contributes to the observed signal at ≈74.018 Da. The signal at +2.00738 likely corresponds to another fragment rather than the ¹³C₂ isotopologue considering the relatively high signal. Other signals (na) are unrelated to the ion of interest. (d−f) simulation of the peak of m + 2 ³⁴S and ³⁰Si isotopologues using the arbitrary example of a monoisotopic mass at 156.04700 Da containing one Si and one S, and three levels of mass resolution. The observed signal (sum) is in grey, while the separate contribution of ³⁴S and ³⁰Si is in blue and orange, respectively. The sampled (i.e., measured by the mass spec) mass that is closest to ³⁴S is circled in red. It shows that there is no sampled mass at exactly +1.995796 (³⁴S mass excess w.r.t. monoisotopic) except at high resolution, and that ³⁴S can be easily confused with ³⁰Si due to their very small mass difference (0.001048 Da). [Color figure can be viewed at wileyonlinelibrary.com]