Mass resolution and isotopic pattern
Mass resolution across the m/z window (50-750 Da) was not found to be
constant, varying from less than 0.0001 Da at low mass (< 100
Da) to 0.0017 Da at higher mass (> 350 Da), i.e. within ≈1
and 5 ppm. In principle, this should allow to solve the natural isotopic
pattern and distinguish 13C, 15N,
and 33S isotopologues (m +1) and13C2 and 34S
isotopologue (m +2). However, this depends on whether analytes of
interest are silylated or not, because Si has two isotopes
(29Si and 30Si), with mass excess
values (with respect to the monoisotopic species) very close to that of33S and 34S (sulphur-containing
species are addressed further below). This is illustrated in Fig. 2,
using a non-silylated compounds (naturally volatile and thus not
requiring derivatisation), isopropyl-4-methylthiazole (IMT). In fact,
using the example of the major fragment (molecule minus
CH3) (Fig. 2a), 15N,33S, 2H and 13C
isotopologues are resolved (Fig. 2b,c). With methionine 2TMS (Fig. 2d),
the presence of Si does not allow full resolution of the isotopic
pattern. This is shown using the m +2 isotopologues (Fig. 2e)
where some isotopic species are found to be undistinguishable
(34S, 30Si). Despite this
limitation, the expected isotopologue abundance (predicted from isotope
abundance and elemental composition) is very close to observed abundance
(Fig. 2f). A difference between observed and predicted abundance is
visible for the deuterated isotopologue of IMT (red circle), likely due
to the very small natural abundance of deuterium (0.015%) and thus the
larger imprecision in quantitation. Also, it should be noted that13C isotopologues are well-separated (resolved) from
other isotopologues, allowing facile monitoring of 13C
molecules during plant labelling experiments, for example.