Isolation and characterization of hydrophilic dissolved organic matter in waters by ion exchange solid phase extraction followed by high resolution mass spectrometry

Dissolved organic matter (DOM) is a complex substance occurring in marine and freshwater environments. DOM has many functions that modify physical, chemical and biological processes in ecological systems. Solid-phase extraction (SPE) is widely used for the separation of DOM. However, a part of strongly hydrophilic compounds are lost using classical SPE separation. In this article, the strong hydrophilic components, which cannot be extracted by the hydrophobic reverse phase SPE, were further extracted in a natural organic matter sample by ion exchange SPE cartridges to obtain hydrophilic acid (HIA), hydrophilic neutral (HIN), and hydrophilic base (HIB) fractions. The extracts were characterized by three-dimensional excitation–emission matrix fluorescence spectroscopy and negative-ion electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The results revealed that the hydrophilic components were significantly different from the hydrophobic components in molecular composition. Hydrophilic components were identified as tryptophan-like and tyrosine-like compounds by three-dimensional fluorescence spectra. The HIA fraction contains mainly tannic acid-like compounds with high O/C ratio; the HIB fraction contains mainly amide-like compounds; and the HIN fraction contains mainly lignin and lipids.