Chromatographic and spectrophotometric properties of dissolved humic substances compared with macromolecular polymers

The commonly accepted polymeric nature of humic substances is still a matte r of debate. In this work, both humic matter of different origin and undisp uted neutral (polysaccharides) and negatively charged (polystyrenesulphonat es) polymers of known molecular weight were studied by high performance siz e exclusion chromatography after dissolution in mobile phases of different composition but constant ionic strength. Modification of the control mobile phase with methanol, hydrochloric acid, and acetic acid showed a progressi ve alteration of the chromatographic behavior of humic materials, but it di d not alter that of polymers. Absolute size reduction of bulk humic materia l revealed by a refractive index detector was accompanied by a substantial decrease in absorbance of peaks indicated by the UV detector. Reduction of molecular absorptivity of humic substances with changes in solution composi tion was confirmed by UV spectroscopy over a wide range of wavelengths. Dif ferences in chromatographic behavior between undisputed polymers and humic samples suggest that humic matter reflects, rather than a polymeric nature, a supramolecular association of heterogeneous molecules held together main ly by weak hydrophobic forces. The content of hydrophilic and hydrophobic c arbons in humic substances appears to control the intermolecular hydrogen a nd dispersive bondings that ultimately stabilize humic conformations in dif ferent mobile phases.