Effect of salinity on molecular characterization of dissolved organic matter using ESI FT-ICR MS

Solid-phase extraction (SPE) coupled with negative-ion Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely used for molecular characterization of dissolved organic matter (DOM). However, little attention has paid to test whether the salinity of the sample and the presence of chloride ions in water samples affect the molecular composition of DOM extracted by SPE (SPE-DOM). In this study, one natural organic matter standard and several natural waters were selected to investigate how salinity affects the molecular composition of SPE-DOM and the selectivity of chloride ion adducts formation with respect to the molecular structure of SPE-DOM in negative ion ESI FT-ICR MS analysis. The results show that the molecular composition of SPE-DOM varied in a sample made by different salinity; and the variation pattern of DOM composition was different among different water samples under the treatment of consistent salinity gradients. The chloride ions can't be completely removed from cartridges in conventional SPE, thus leading organic compounds in SPE-DOM to form [M+Cl]‾ adducts when performing ESI FT-ICR MS analysis. In addition, the molecules with high H/C and low O/C ratios were likely to form [M+Cl]‾ ions. The relative abundance of [M+Cl]‾ ions could increase with the increase of salinity. These results are instructive to guide the pretreatment and molecular characterization of DOM in water samples with different salinity. Overall, we proposed a modification to the SPE to minimum reduce the formation of chloride ion adducts during the isolation of DOM.