Compositional differences between size classes of dissolved organic matter from freshwater and seawater revealed by an HPLC-FTIR system. 
The molecular complexity of dissolved organic matter (DOM) hinders its characterization.  New approaches are thus needed for a better understanding of DOM reactivity and fate in aquatic systems.  In this study, high-performance liquid chromatography (HPLC), using size-exclusion separation, was coupled with Fourier transform infrared spectroscopy (FTIR).  A solvent-elimination interface was used to deposit DOM fractions onto a germanium disk that were then analyzed by FTIR.  Samples included ultrafiltered DOM (UDOM) and fulvic acids from the St. Lawrence Estuary and its tributaries.  Results showed significant compositional changes with molecular size and origin, especially in UDOM.  Larger fractions of UDOM contained more carbohydrates, amides, aromatics/alkenes and aliphatics, while smaller fractions contained more carboxylate and OH groups.  Small marine molecules (500-900 Da) were also enriched in sulfate groups that appeared bound to UDOM.  Large marine molecules were the most amide-rich fractions.  Fulvic acids were enriched in carboxylate and OH groups, showed little changes in composition, and appeared similar to small terrigenous (riverine) UDOM even in marine water.  This work shows that an HPLC-FTIR system is a powerful and complementary tool in the characterization of DOM.  The compositional changes observed may explain the reported contrasting reactivity and fate of DOM having different size and origin.