Fig. 2. Isotopic pattern in two N- and S-containing molecules,
isopropyl-4-methylthiazole (IMT) and methionine 2TMS . (a) overall mass
spectrum of IMT, showing the prevalence of the demethylated fragment at
126.03770 Da. (b) magnification of the mass spectrum showing the
isotopic pattern of the major fragment of IMT and (c) detailed view ofm +1 isotopologues showing the resolution of15N, 33S, 13C and2H isotopologues. (d) overall spectrum of methionine
2TMS with two major fragments (128.08955 Da and 176.09290 Da) and (e)
magnification showing m +2 isotopologues. (f) relationship between
expected and observed relative abundance of isotopologues in IMT using
two fragments (blue, green) and methionine 2TMS (pink), plotted using a
log scale. Note the underestimation of deuterium (2H)
content (red circle). The black line stands for the 1:1 line. In (a) and
(d), fragmentations are shown with chemical formulas to illustrate how
the fragments of interest are formed. Note IMT does not require
derivatisation while methionine is silylated (two TMS groups). In (e),
note that there is a common peak for 34S (+1.995796
Da) and 30Si (+1.996844 Da) and also33S (+0.999387 Da) and 29Si
(+0.999568 Da).