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).