Characterization of soil humin by pyrolysis(/methylation) gas chromatography mass spectrometry: Structural relationships with humic acids

Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and Py-GC/MS in the presence of tetramethylammonium hydroxide (TMAH) (pyrolysis/methylation-GC/MS) have been used to characterize humin of a representative agricultural soil. Analyses have been performed on humin de-ashed by treatment with HCl-HF (HU) and the results have been compared with those obtained from the corresponding humic acid (HA). Both semiquantitative conventional pyrolysis and pyrolysis/methylation agreed in showing that the pyrolytic behaviour of HU is quite different from that of HAI the mass spectrometry pyrograms of the former being characterized by a larger amount of carbohydrates and aliphatic compounds. Pyrolysis/methylation of HU produced methyl esters of aliphatic and aromatic carboxylic acids, methylated carbohydrate derivatives and methyl pyrroles. Peaks diagnostic of the presence of glucose and cellulose have been proposed when pyrolysis is performed in the presence of TMAH.