Enhanced Structural Understanding of Dissolved Organic Matter through Comparative LC/MS2 Analysis with Synthetic...

Dissolved organic matter (DOM) is one of the most complex chemical mixtures known, with its chemical composition having long puzzled biogeochemists. Identifying the chemical structures within DOM is essential for unraveling its origins and environmental fate. However, DOM's complexity has impeded structural elucidation, and molecules with accurate functional group compositions for recalcitrant DOM are poorly represented in the synthetic and isolative literature. Consequently, hypothesized DOM compounds are derived from models that inadequately represent true structures. To address this, carboxylic-acid-only CRAM analogues were previously synthesized but failed to replicate the extensive fragmentation observed in marine DOM during tandem mass spectrometry (MS2). Here, we prepared CRAM analogues with varied oxygen functionalities to enable more diverse fragmentation pathways. Liquid chromatography (LC) studies showed that functional group composition better predicted LC polarity than the O/C ratio and that alcohols represented early eluting DOM profiles, while ethers, ketones, and lactones better represented mid-eluting isomers. MS2 studies revealed that the incorporation of α-hydroxy ketones and 1,2-diols led to the most extensive fragmentation. Ether and ester functionalities were labile even at low fragmentation energy, indicating that such groups are likely contributors to core marine DOM carbon backbones and contribute to the extensive fragmentation observed for natural DOM in all MS2 experiments. The data gathered within this work suggest that the widely discussed all carbon-backbone alicyclic model of CRAM is incompatible with the MS2 fragmentation data of DOM.