Identification of S-containing analytes
The resolution of the isotopic pattern can be exploited to gain information on uncommon (or less common) elements contained by analytes. Nitrogen 15N is not sufficiently abundant (0.36% only) and can be overlapped by 29Si (4.7%) and33S (0.76%) isotopes. Therefore, the use of the isotopic pattern to detect specific elements in analytes is only suitable for sulphur, provided (i ) the signal coming from34S is high enough (34S abundance is 4.2%), and (ii ) it can be somehow distinguished from30Si. The mass excess of the 34S isotopologue is +1.995796 while that of 30Si is +1.996844, meaning a difference of 0.001048 Da (i.e., 1.048‰). If mass resolution during analysis is sufficient, mass excess at nearly exactly +1.995796 can be found. A similar analysis with high resolution LC-MS (without the issue of Si isotopes) has been undertaken previously (Nakabayashi & Saito, 2017). In Fig. 5, automatic searching of mass excess of +1.995796 is shown, with the difference (in ‰) as a function of feature m/z. Using a mixture of authentic standards that includes methionine and cysteine (Fig. 5a), only 9 (out of more than 2,000) features were found to fall in the mass excess +1.995796±0.00005 (red shaded area). From these, manual checking allows confirmation of 3 of them, another one (labelled as 4 in Fig. 5a) being ambiguous:1 and 2 are S-containing fragments of cysteine 2TMS (61.011195 and 131.035071 Da at 5.61 min), while 3 is a fragment of methionine 2TMS (176.092921 at 9.12 min). 4 appears at the retention time of methionine 2TMS but in the spectrum, the peak is too wide to ascertain that it is not another fragment with30Si (i.e., other fragments without S with nearly the same exact mass). Using a plant sample (Arabidopsis leaf extract, Fig. 5b), about 10 features fall in the ±0.05‰ window, of which two can be confirmed further: 1 is the same as in Fig. 5a (S-containing fragment of cysteine 2TMS) and 5 is C3H6S (74.018421 Da) and comes from methionine 2TMS, at 9.12 min. The appearance of this feature is further illustrated in Fig. 5c, where it is found to be very close to the isotopic pattern of another methionine 2TMS fragment (C2H6OSi). Of course, looking at the mass excess (due to 34S) with a high precision in Figs. 5a and 5b is only possible if resolution is high enough to have a better probability to have an acquired m/z datapoint exactly at, or very close to +1.995796. This is illustrated in Fig. 5d-f, where the observed signal (grey) is likely to show a mass signal coming from34S at medium and high resolution. This can optimally be achieved for relatively small features (m/z < 150 a.m.u.) for which resolution is larger.