Structural information obtained from spectral analysis (UV-VIS, IK, 1H NMR) of particle size fractions in two humic acids

Humic acids (HAs) extracted from an Umbric Andosol (UA) and a Dystric Cambisol (DC) were fractionated into 7 fractions by successive gel permeation chrontatography (GPC) and their fractions were studied by spectral analysis. With increasing particle sizes, the absorption of methylene groups and amide of peptides increased in the IE spectra of both HAs and that of carboxylic groups decreased in the IR spectra of UA, the HA obtained from an Umbric Andosol. Based on the 1? NMR data in UA, there was a significantly negative linear correlation (−0.950, p< 0.001) between Har (δ 6.0-9.0 ppm) and HC-O (δ 3.0-4.3 ppm). Such a negative linear correlation (−0.868, p<0.01) was also observed between Har and Hβ (δ 0.9-1.6 ppm). In addition, with increasing particle sizes, the percentage of aromatic protons decreased (from 30.6 to 4.2%). These results suggest that the increase in HA particle sizes depends on the length of aliphatic groups and the bond of methoxyl groups, and HAs were composed of HA molecules of homologous series. On the other hand, in DC, HA obtained from Dystric Cambisol, no significant relationships were observed between each type of protons and aromatic protons. It is suggested that a mixture of non-humic substances smeared the spectral data of DC.