Chemical characterization of humic substances using Thermochemolysis with tetramethylammonium hydroxide

Póster presentado en el 18th International Meeting on Organic Geochemistry. Part I. Abstracts, 22-26 septiembre 1997 (Maastricht) The NetherlandsA considerable number of analytical approaches have been used to characterize the structural composition of humic substances (HS). Pyrolytic techniques, in combination with gas chromatography and mass spectrometry (Py-GC-MS), provide detailed structural intonation at the molecular level and have been extensively applied to unravel the structure of HS. Recently, a structural model for HS has been proposed on the basis of mainly pyrolysis data (Schulten et al., 1991); however, it has been criticized heavily (de Leeuw and Hatcher, 1992; Saiz-Jimenez, 1994) because of the inherent problems associated with The analytical pyrolysis of highly polar substances such as HS. It is known that some structural moieties known to be present in HS can be heavily modified by unwanted Thermal reactions which may lead to structural misinterpretations. More specifically, benzenecarboxylic acids and longchain fatty acids be decarboxylated and transformed lo other products upon pyrolysis (Martin et al., 1994; Saiz-Jimenez, 1994). Also, very polar products are often not volatilized and escape detection by gas chromatography. Pyrolysis in The presence of tetrarnethylammonium hydroxide (TMAH) was introduced to overcome the limitations of conventional pyrolysis. This procedure produces the methyl esters of carboxylic acids and methyl others of hydroxyl groups, preventing decarboxylation and rendering many of the polar products volatile enough for gas chromatographic analysis. In particular, the application of pyrolysis/TMAH to humic materials have confirmed the presence of benzenecarboxylic acids and fatty acids in the structural makeup of HS. Based on Ibis information, it is clear that structural models based largely on Py-GC-MS studies should be re-evaluated (Saiz-Jimenez, 1994; Hatcher and Clifford, 1994; Martin et al., 1994; del Rio et al., 1994). In some recent papers (McKinney et al., 1995; Hatcher et al., 1995), a new off-line procedure was introduced for the depolymerization and characterization of lignin al subpyrolysis temperatures in the presence of TMAH using sealed glass tubes, the so-called TMAH/thermochemolysis procedure. Hatcher and Clifford (1994) showed that reducing the temperature of the pyrolysis/TMAH from 600°C to 300°C did not drastically alter the product distribution for HS. This indicates !hat The TMAH reaction is mainly a Thermochemolysis and that it can be performed in an offline fashion in sealed glass ampoule in a manner analogous to that described by McKinney et al. (1995).N