Joanna Miklewska, Department of Land Use and Rural Development, University of Agriculture, 70-466 Szczecin, Poland

  •   1.  Introduction

    Humic substances are macromolecular organic compounds with heterogenous monomers and heterogenous kinds of linkages formed by random events. Therefore a reproducible structure or exact structural formula are impossible in this case (Stevenson 1982, Kumada 1987, Ziechmann 1988, Saiz-Jimenez 1996). Various experimental techniques were applied in studies on humic substances, still their composition and chemical structure seem to be only statistically defined. One of widely applied yet often not fully utilized non degradative methods is UV and visible spectroscopy.

    It is widely known that UV visible absorption spectra of soil humic acids are usually monotonously decreasing with increasing wavelength and thus difficult to interpret (Stevenson 1982, Kumada 1987). Various coefficients based on ratios of absorption at given wavelengths were often used in studies of humic substances (Chen et al. 1977, Sławiński et al. 1978, Kulchaev 1980,  Sugahara et al. 1984, Baes and Bloom 1990, Wang et al. 1990, Amalfitano et al. 1995) . However they usually served merely as additional indexes characterizing humic materials (Choudhry 1981, Saiz-Jimenez 1996). Absorption spectra of humic acids (HAs) in the visible region seem to have been largely ignored because of their featureless character. But this very featurelessness is induced by the brown color of HAs, which is a part of their definition (Kumada 1965). Thus it seems reasonable to look for some structural information on humic acids hidden in their apparently featureless spectra. Presence of various organic moieties in the structure of humic acids should be reflected in their absorption spectra.

    Transformation of monotonic absorption spectra of humic acids in their fourth derivatives results in apparent absorption bands, which reality has been proved with mathematical modeling elsewhere (Miklewska 1996). The method was earlier applied to resolve UV spectra of for example proteins (Siemenova and Saakov 1989). This paper presents a preliminary results of differentiation of UV visible absorption spectra of humic acids.