Differentiation of metabolites with exact mass
Exact mass allows one to distinguish m/z features with the same nominal
mass but with very slight differences (well below 0.1 Da), reflecting
differences in elemental composition. This is particularly useful when
retention time of analytes at the origin of similar m/z features are
very close. We took advantage of GC-MS exact mass analyses of authentic
standards used to construct the database (available as a Supplementary
Files), to tackle this problem. Amongst most common plant metabolites,
many analytes were found to have both similar retention time and
identical nominal mass m/z signals, and they are listed in Table 1. This
problem can potentially lead to identification mistakes with nominal
mass analyses especially when mass spectra are rather similar. Two
examples are further illustrated in Fig. 3. Ferulic acid 2TMS and
tyramine 2TMS have the same retention time (12.09 min) and share the
same nominal mass ion 338 a.m.u. However, there is a clear difference in
exact mass (Fig 3a,b). Also, their spectra share little similarity (Fig.
3a). Malic acid 3TMS and threitol 4TMS have a retention time difference
of less than 4 s and share a feature at a nominal mass of 189. In
addition, their mass spectra are partly similar, in particular for
features < 200 a.m.u (Fig. 3c). However, the exact mass of
features at 189 a.m.u. is readily distinguishable (189.11309 and
189.076741 Da) (Fig. 3d).