Effect of off-line methylation using carbanion and methyl iodide on pyrolysis-gas chromatographic analysis of humic and fulvic acids

The effect of off-line alkylation using a powerful methylating reagent of carbanion and methyl iodide on the kind and yield of degradation products of humic and fulvic acids in their pyrolysis-gas chromatography (Py-GC) was examined. The humic and fulvic acids used were prepared from three soils including a Dystric Cambisol, an Umbric Andosol, and a Podzol. The yields of aliphatic and lignin-derived phenolic compounds did not increase or decreased generally after the off-line methylation in comparison with conventional thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxide (TMAH) for unmethylated samples. However, benzenetetracarboxylic acid tetramethyl esters and benzenepentacarboxylic acid pentamethyl ester became detectable or increased in yield for all the three fulvic acids examined and one humic acid sample characterized by its high concentration of carboxyl groups. Benzenehexacarboxylic acid hexamethyl ester was also detected in off-line methylated fulvic acids soluble in dichloromethane and purified with Sephadex LH-20. The addition of TMAH in pyrolysis of the off-line methylated samples did not improve the yields. Hence, the conventional THM comparatively lead to lower yields of the benzenepolycarboxylic acids (BPCAs) with 4–6 carboxyl groups, and the Py-GC after the off-line methylation could give additional information on structural unit of humic substances, although its single use did not increase total yields of degradation products.