Increasing the analytical window of marine dissolved organic matter for orbitrap mass spectrometry

Marine dissolved organic matter (DOM) is a major carbon pool comparable to the atmospheric carbon pool and serves important roles in long term carbon storage. Elucidating the chemical composition of DOM is necessary to improve our understanding of its roles in global biogeochemical cycles. Most studies utilize solid-phase extraction (SPE) to concentrate and desalinate samples, followed by high-resolution mass spectrometry to characterize DOM composition. The conventional SPE method achieves up to 70% dissolved organic carbon (DOC) extraction efficiency; however sample acidification prioritizes the extraction of organic acids creating an incomplete chemical profile. Although a robust methodology, many compounds are not retained by the non-polar SPE resin constraining the analytical window creating a hidden fraction of DOM. This study aims to generate a method to elucidate the DOM fraction previously uncharacterized by targeting DOM constituents with basic functional groups. Surface seawater samples were processed by two distinct SPE procedures; the conventional acidified SPE procedure and the modified acid-free procedure. Extracts were analyzed with ion chromatography and liquid chromatography coupled to a high resolution, mass accuracy Orbitrap Fusion Tribrid mass spectrometer. Samples were analyzed by data-dependent acquisition in negative and positive modes, and databases were used for tentative structural identification of Individual DOM compounds. DOC extraction efficiencies of the acid-free SPE method were around 40%, even though it recovered lower DOC; still, it shows significant chemical composition differences relative to the conventional SPE procedure, indicating it extracts a distinct DOM fraction. These results suggest that utilizing both the conventional SPE procedure and the acid-free SPE procedure on seawater samples provide a more comprehensive chemical profile of marine DOM, overcoming the current bias towards acidic constituents. This method better represents marine water samples and has the potential to become a routine method for future marine DOM characterization.