Characterization of estuarine and fluvial dissolved organic matter by thermochemolysis using tetramethylammonium hydroxide

Selected samples of dissolved organic matter (DOM) isolated by ultrafiltration (UDOM) have been analyzed by thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH). This technique cleaves ester and ether bonds of bio- and geological macromolecules and releases monomer subunits and methylates them in situ as their methyl ethers and methyl esters. Compared with conventional pyrolysis, TMAH thermochemolysis avoids decarboxylation of preexisting carboxylic moieties and produces aromatic acids as their methyl esters. Various phenolic derivatives, which might originate from incorporated lignin-derived structures, from the highly aliphatic and resistant biopolymer cutan and also from proteinaceous materials, were identified among the products produced from UDOM upon thermochemolysis. The presence of lignin derivatives in UDOM indicates input of organic matter derived from terrestrial sources. Various aromatic acids, perhaps representing the final steps in the oxidation of the side-chain during microbial oxidation of lignin, were released upon TMAH thermochemolysis, suggesting they are structural constituents of the UDOM. Different ratios of lignin-derived materials, commonly determined using the CuO oxidation method, such as the Lambda value, indicative of the amount of lignin present, the acid/aldehyde ratio (Ad/Al)(G), indicative of the extent of oxidative degradation of the lignin component, and the syringyl/guaiacyl (S/G) and p-hydroxyphenyl/guaiacyl (P/G) ratios, indicative of the contribution for the different types of lignin, were determined.