Vol. 9, No. 1, 13-21, 2010

Abstract
Humic substances (HS) play important roles in a variety of biogeochemical processes. Fluorescence spectra can be used for quantitative and qualitative characterisation of water-soluble humic substances (HS) and, in particular, of dissolved organic matter (DOM) in water. In this study, we measured the fluorescence spectra of natural HS (riverine, lake, marine water, and soil aqueous extractions) and commercial water-soluble HS under excitation from 270 to 355 nm. Natural water samples were filtered in two stages by micro- and ultra-filtration. After ultra-filtration, DOM was separated into two fractions depending on the size of the particles, for which fluorescence spectra were also obtained. The comparative analysis was made with a focus on the so-called "blue shift" (emission maximum shifts towards shorter wavelengths with increasing excitation wavelength) and the fluorescence quantum yield (QY) of samples of different origin. The fluorescence quantum yield under excitation at 355 nm for commercial HS (QY=0.008) was less than that for natural water DOM (QY=0.028), but more than for soil HS (QY=0.003). The low molecular weight fraction quantum yield was bigger than that for the colloidal fraction by 20-30%. The fluorescence quantum yield for natural water and for soil extractions HS increased with excitation wavelength, but the quantum yield did not depend on the excitation wavelength for commercial HS samples. Natural HS differed from commercial HS in higher heterogeneity of fluorophore composition as evidenced by the larger "blue shift" value and QY dependence on excitation wavelength.

ISSN 1729-3782