Semiquantitative GC/MS analysis of thermochemolysis products of soil humic acids with various degrees of humification

Abstract

Thermochemolysis with tetramethylammonium hydroxide (TMAH) was conducted for 18 humic acids (HAs) obtained from a wide range of soils, and the yields and compositions of the released compounds were analyzed in terms of the degree of humification and the proportions of C species estimated by 13C CPMAS NMR. Eighty-six peaks could be assigned on GC/MS chromatograms, and notable differences in the kind of compound assigned were not found among the HAs. The total yields of aliphatic compounds ranged from 0.40 to 4.2 mg/g. C 12- to C 18-even fatty acid methyl esters (FAMEs) accounted for 30–66% of the entire aliphatic compounds assigned, followed by C 20- to C 34-even FAMEs (4.8–47%), C 9- to C 33-odd FAMEs (11–26%), and C 16- to C 30-even dicarboxylic acid dimethyl esters (DAMEs; 0.3–9.1%). The total yields of aliphatic compounds decreased with decreasing gross alkyl C during the progress in the degree of humification, although their proportion in gross alkyl C was small. On the other hand, the total yields of aromatic compounds ranged from 1.7 to 37 mg/g. Of these, 7.5–82% and 0.12–64% were phenol derivatives that were possibly derived from lignin and benzenepolycarboxylic acid polymethyl esters, respectively. The yields of phenol derivatives and stilbene compounds, possibly originating from lignin dimers, decreased with an increase in the degree of humification, contrary to the increase in the proportion of aromatic C in total C. The yields of heterocyclic compounds, namely, indole and proline derivatives, were also higher in HAs with lower degrees of humification. The variation in the yield of benzenepolycarboxylic acid polymethyl esters suggested parallel progression of oxidation with increasing degree of humification. Aromatic compounds responsible for the high aromaticity and the dark color of HAs, such as quinones and polycyclic compounds, were not detected even in HAs with a high degree of humification.