Fig. 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 as30Si 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 13C2isotopologue considering the relatively high signal. Other signals (na)
are unrelated to the ion of interest. (d-f) simulation of the peak ofm +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 with30Si due to their very small mass difference (0.001048
Da).