Laser diagnostics of various types of humic substances using fluorescence saturation spectroscopy
E.M. Filippova (1), T. A. Zubkova (2), L. A. Karpachevski (2), V. V. Fadeev (1)
(1) Quantum Radiophysics Division, Department of Physics
(2) Physics and Melioration of Soil Division, Department of Soil Science,
Moscow State University, 119899 Moscow, Russia
Humic substances (HS) are very important compounds of aquatic and land environment that provide the normal conditions for co-existence of every living thing in the biosphere. This ubiquitous matter was found in all types of water, soil and sediment. HS serve as the main link in global cycle of organic carbon, contain the basic reserve of organic carbon on the Earth. Luminescence is the inalienable property of humic matter, so fluorescence spectroscopy is widely used for diagnostics. Diagnostic means the simultaneous solution of two problems: (1) identification of different HS kinds and (2) determination of the HS concentration (as an example, in units of organic carbon concentration). Identification problems cannot be solved only by using traditional spectral analyses (comparison of the bandshapes, half-widths of fluorescence excitation and emission spectra, maximum positions, and other). Aquatic, soil and sediment HS have very similar spectra, although the origins and genesis of these substances are different. Phytoplankton and land vegetation are the main producers of organic material in the ocean and soil, respectively. The quantum yield of fluorescence and connection coefficient between the fluorescence intensity and the HS concentration also differ for various HS. So, humic matter samples should be identified for reliable diagnostics.
In this paper, the possibilities of fluorescence saturation spectroscopy for HS identification are presented. Saturation technique allows one to determine the following HS molecular parameters: (1) the effective excitation cross section of fluorescence states in a whole complex; (2) the constant of singlet-singlet annihilation. The values of these parameters depend on structure, composition, intermolecular interactions and characterise different kinds of HS organic complexes. Some experimental results, the molecular HS parameters, and the algorithm for solution of inverse problem of saturation spectroscopy are presented in this paper.