CuO-oxidized dissolved organic matter (DOM) investigated with comprehensive two dimensional gas chromatography-time of flight-mass spectrometry (GC × GC-TOF-MS)

GC × GC-TOF-MS was applied to investigate the molecular diversity of CuO-oxidized, trimethylsilyl (TMS) derivatized DOM isolated from Lake Tahoe's main tributary, the Upper Truckee River (UTR), California. Many (4789) chromatographic peaks were resolved in a single sample at a signal/noise (S/N) ratio of 10 or greater and > 300 (> 6%) were assigned discrete structures. Mapping the two dimensional elution time space revealed 8 homologous series defined by successive CH2 additions. Series of other compounds related to one another by OCH3 and OH substitutions were also assigned. Elucidation of the retention time (RT) displacements affected by these molecular transformations guided the discovery of novel compounds as well as those that had previously escaped detection within DOM, including a suite of lignin-derived cyclobutane photodimers. Analysis of RT shifts among sets of benzene polycarboxylic acid (BPCA) isomers revealed the second chromatographic dimension to strongly retain sterically strained isomers, providing a basis for assigning groups of isomers that were otherwise unassignable. The heightened chromatographic resolution also revealed a substantial and hitherto analytically inaccessible diversity of isomers, which appear to be uniquely resolved with the method, but which would fall outside the analytical window of other high resolution MS methods, such as Fourier transform ion cyclotron resonance MS (FT-ICR-MS). The study likely represents the most comprehensive compound-specific elucidation of any natural OM (NOM) sample to date.