The ratio of optical densities or absorbances of dilute, aqueous humic and fulvic acid solutions at 465 and 665 nm (E4/E6) is widely used by soil scientists for the characterization of these materials. While it has been suggested that the E4/E6 ratio is related to the degree of condensation of the aromatic carbon network, carbon content, and molecular weight of humic substances, little rigorous experimental evidence is available in the literature to confirm these hypotheses.The results of this investigation show that the E4/E6 ratio of humic and fulvic acid is: (i) mainly governed by the particle size (or particle or molecular weight); (ii) affected by pH; (iii) correlated with the free radical concentration, contents of O, C, CO2H and total acidity in as far as these parameters are also functions of the particle size or particle or molecular weight; (iv) apparently not directly related to the relative concentration of condensed aromatic rings; (v) independent of humic acid and fulvic acid concentrations, at least in the 100–500 ppm range. Our data show, in agreement with M. M. Kononova (1966), that E4/E6 ratios for humic and fulvic acids should be determined between pH 7 and 8. This can best be done by dissolving the humic material in 0.05N NaHCO3 solution at concentrations of 200–400 ppm.

Micro-FTIR mapping is a powerful tool for nondestructive, in situ chemical characterization of coal macerals at high resolution. In this study, the chemistry of resinite, funginite and associated vitrinite is characterized via reflectance micro-FTIR for Cenozoic high volatile C bituminous coals from Colombia. In comparison with the micro-FTIR spectra of vitrinite and inertinite, the corresponding spectra of liptinite macerals in the same coals are characterized by stronger aliphatic CH(x) absorbance at 3000-2800 and 1460-1450 cm⁻¹, but less intense aromatic C=C ring stretching vibration and aromatic CH(x) out-of-plane deformation at 700-900 cm⁻¹. The aliphatic components in resinite have the longest carbon chains and are least branched, bestowing the highest hydrocarbon generation potential on resinite among the three macerals studied. In contrast, funginite exhibits the strongest aromatic character, the highest aromaticity, the lowest 'A' factor values and the lowest C=O/C=C ratios among the three maceral groups. Vitrinite generally displays intermediate chemical characteristics. Reflectance micro-FTIR mapping of coal samples further confirms the aliphatic character of resinite and the aromatic nature of funginite. In addition, chemical mapping of resinite and adjacent vitrinite shows that vitrinite immediately adjacent to resinite displays higher aliphatic CH(x) stretching intensity than more distant vitrinite, suggesting that chemical components from resinite can diffuse over short distances into adjacent vitrinite, specifically causing hydrogen enrichment. It needs to be pointed out, however, that the region of influence is localized and limited to a narrow zone, whose extent likely depends on resinite's properties, such as its size and aliphatic material content. This way, the chemical map of resinite and associated vitrinite provides direct evidence of the intermaceral effects occurring during the peat forming stage or during later coalification. No influence of funginite (primarily fungal spores and sclerotia) on the chemistry of adjacent vitrinite has been demonstrated, which is likely due to the highly aromatic structure of this type of funginite.