Fractionation and molecular characterization of natural organic matter (NOM) by solid-phase extraction followed by FT-ICR MS and ion mobility MS

Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely used for the characterization of dissolved organic matter (DOM) extracted by solid-phase extraction (SPE) from various environmental waters. It is known that common SPE generally has a relatively low recovery and the non-polar and weakly polar components are less likely to be ionized due to ionization discrimination. However, the molecular selectivity during SPE is not clear so far. In this study, the Suwannee River natural organic matter (SRNOM) was fractionated by multi-step SPE with different solvents and the fractions were characterized by negative ESI FT-ICR MS and trapped ion mobility spectrometry-mass spectrometry (IMS-MS). The sequential solvent elution increased the extraction recovery of DOM in water by SPE and enabled the characterization of a weakly polar component, which cannot be detected in common SPE separation. The weakly polar component accounts for 5.7% in TOC for the SRNOM, which has complex but different molecular composition with methanol- and/or water-eluted fractions. Lipid-like compounds were enriched in this fraction. Compared with the polar molecules directly eluted by one-step SPE from the SRNOM, the weakly polar fractions exhibit larger molecular size. The results are helpful for understanding of the molecular composition of SRNOM, as well as other environmental waters. In addition, the study demonstrates that the combination of FT-ICR MS and IMS-MS has potential to characterize the chemical composition of a complex mixture, like DOM, to a new depth.