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.