Laser fluorescence spectroscopy as a method for studying humic substance

This paper summarizes the results obtained by fluorescence spectroscopic te chniques with conventional and laser light sources for the characterization of the spectral luminescent properties of aquatic humic substances (AHS). The band profiles of corrected absorption, fluorescence emission, and fluor escence excitation spectra are compared in graphical form with unique funct ions used for the axes. In order to calculate the quantum efficiency of flu orescence, we used the Raman signal from water molecules as an internal ref erence. The temperature dependence of the fluorescence spectra of humic mat ter has been recorded in solid (-160 degreesC to 0 degreesC) and in liquid (0 degreesC to +300 degreesC) aqueous solutions. The behavior of the fluore scence band shape is discussed. A new method of nonlinear fluorimetry is pr oposed to enhance the capabilities of fluorescence spectroscopy. This metho d is based on the fluorescence saturation effect manifesting itself under p owerful laser pumping (excitation) conditions. The use of the technique all ows one to determine certain photo-physical properties of molecules, as a c omplement to the normal spectral data. Using three different samples of aqu atic humic substances with very similar fluorescence band shapes, it is dem onstrated that these compounds have distinct molecular luminescent paramete rs and therefore. can be identified. The similarity of the fluorescence ban d shapes of humic substances in natural water of different types; the absen ce of any noticeable effect of temperature across a wide range and the fluo rescence saturation on the fluorescence band contour can be explained. This assumes that in all of the samples of aquatic humic substances studied, wi th different molecular weight fractions, that there is a single dominant fl uorescent functionality present.