EARSeL eProceedings 9, 1/2010 13 THE “BLUE SHIFT” OF EMISSION MAXIMUM AND THE FLUORESCENCE QUANTUM YIELD AS QUANTITATIVE SPECTRAL CHARACTERISTICS OF DISSOLVED HUMIC SUBSTANCES Daria Shubina 1 , Elena Fedoseeva 2 , Olga Gorshkova 3 , Svetlana Patsaeva 1 , Vera Terekhova 4 , Mikhail Timofeev 5 and Viktor Yuzhakov 1 1. Moscow State University, Department of Physics, Moscow, Russia; daria.shubina(at)gmail.com, spatsaeva(at)mail.ru 2. Irkutsk State University, Department of Biology and Soil Studies, Irkutsk, Russia 3. Moscow State University, Department of Geography, Moscow, Russia 4. A. N. Severtsov Institute of Ecology and Evolution RAS, Moscow, Russia 5. Moscow State University, Department of Soil Science, Moscow, Russia ABSTRACT Humic substances (HS) play important roles in a variety of biogeochemical processes. Fluores- cence 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 quan- tum 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. INTRODUCTION The goal of this study is to examine the spectral characteristics of humic substance (HS) from natural water and soil. HS plays an important role in biogeochemical processes. Up to 50% of dis- solved organic matter (DOM) in natural water and about 12% of soil organic matter is HS (1). In spite of a two-century history of HS research (2,3), there are a lot of unsolved problems concern- ing its structure and properties. HS absorbs in the UV and in the visible (blue light) range of the spectrum. Therefore, natural water containing a large amount of HS has yellow colouring and soils with large amounts of HS are brown and black (4). HS is particularly suitable for spectral research since its complex structure precludes analysis by chemical methods. Macromolecules of HS have a chaotic structure, which can be investigated by methods considering total ensemble properties such as luminescence spectroscopy. Moreover, HS is an interesting object for spectroscopy as an aggregate of interacting fluorophores. Such a mac- romolecular aggregate could vary in size from a few nanometres to micrometres. HS dissolved in water has a maximum fluorescence in the visible range under excitation at UV wavelengths. Fluorescence spectra can be used for quantitative and qualitative characterisation of water-soluble HS fractions. The most extensively studied DOM fluorescence spectra are those of DOM in natural waters (5,6,7,8). The wide band of DOM emission has a fluorescence maximum in