Resolving dissolved organic matter : new multidimensional chromatographic approaches

This thesis focuses on the study of dissolved organic matter (DOM) as one of the earth's largest carbon reservoirs and one of the most complex naturally occurring mixtures of organic material. DOM is a critical component on biogeochemical processes which relevance, origin, fate and chemical composition, which are still relatively poorly understood. In chapter 2, a detailed review on the on the extraction, fractionation and chemical characterization of dissolved organic matter (DOM) from seawater and freshwater sources for the rapid and effective processing of this complex sample prior to mass spectrometry (MS) and nuclear magnetic resonance (NMR) analysis is presented. This thesis also explores new single- and multidimensional chromatographic approaches to the fractionation of DOM. Within chapter 3, a relatively new chromatographic method, high-performance counter-current chromatography (HPCCC), was applied to the study of DOM. A HPCCC method was developed for the quantification of solid phase pre-extracted low molecular weight dissolved organic matter (DOM) from natural water sources. The method was applied to the determination of the concentration of DOM in seawater and compared with traditional quantification, demonstrating a clear advantage for the determination of the amount of DOM in water by using small volumes of samples. Detailed within chapter 4, a new separation technique based on Eleven Onyx monolithic C18 columns connected in series was developed in order to obtain a high capacity reversed- phase HPLC column providing 110,000 theoretical plates for the fractionation of the components of DOM. The method was complemented by coupling the main fractionation with a second dimension of reversed-phase HPLC and high-resolution mass spectrometry (HRMS) detection. Successful fractionation of the major compositional materials within DOM (i.e. carboxylic-rich alicyclic molecules, CRAM) in order of decreasing polarity was confirmed. This research also seeks to provide improve pathways to the characterization of DOM by addressing the extraction of DOM from fresh and marine waters by using three different solid phase extraction (SPE) adsorbents, the selectivity of, phenyl-hexyl functionalised silica gel, a novel in house prepared adsorbent, and the commercially available polystyrene di- vinyl-benzene (PS-DVB), and octadecyl-silica gel (C-18) based adsorbents was described in chapter 5. Compositional differences between DOM extracted using the three different types of adsorbents could clearly be seen. DOM obtained from phenyl-hexyl functionalised silica proved to be richer in aromatics, aldehydes and aliphatics molecules than the other adsorbents. Finally, within chapter 6, for the first time, the characterization of dissolved organic matter (DOM) from seafoam samples extracted via solid phase extraction SPE using C18 and PPL functionalised adsorbents was performed. The results highlight the different selectivity of the examined adsorbents and underlining the potential to isolate specific classes of compounds within complex mixtures such as seafoam. Furthermore, NMR and HRMS analysis of the seafoam samples, confirmed the presence of the following classes of compounds: aliphatics, unsaturated, aromatics, aldehydes, peptides and carbohydrates in high concentrations within DOM.