Preliminary Study of Lake Dissolved Organic Matter in Light of Nanoscale Supramolecular Assembly

Abstract

Large-scale preparative high-performance size-exclusion chromatography (HPSEC) was performed to separate different molecular size fractions in milligram quantities from strongly colored dissolved organic matter (DOM) of a freshwater using a very mild conjugate acid-base pair (10 mM acetic acid-sodium acetate solution at pH 7.0 with an ionic strength of 6 x 10(-5)) as the mobile phase. The homogeneity-uniformity of different molecular size fractions in relation to their combined original mixture was verified by an analytical HPSEC system. In addition to molecular size distribution and basic spectroscopic characteristics, Fourier transform infrared spectroscopy was applied to specify structural features for different size fractions. The results demonstrate clearly that only a very small amount of conjugate organic acid-base pair is required to generate a powerful resolution for a DOM mixture, and very strong treatments with organic acids are not necessarily needed to reach a better SEC resolution. Most essential is the combined outcome of different HPSEC experiments and determined structural functionalities which indicate that almost all original DOM solutes are aggregated mixtures consisting of different associations possessing various molecular size ranges, which can be separated from their integrated whole as nearly homogeneous and uniform species. In summary, the present study strongly speaks for the need to direct the research of natural dissolved and colloidal organic carbon more strongly toward a nanoscale study of supramolecular assemblies. More precise knowledge about the primary, secondary, and tertiary structure of dissolved DOM constituents has its essential function, e.g., for environmental protection and utilization of surface waters.