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Faculty for Biology, Chemistry and Earth Sciences

Department Soil Ecology - Prof. Dr. Eva Lehndorff

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Kalbitz, K; Geyer, W: Humification indices of water-soluble fulvic acids derived from synchronous fluorescence spectra - effects of spectrometer type and concentration, Journal of Plant Nutr. Soil Sci., 164, 259-265 (2001)
The high variability of dissolved organic matter (DOM) in natural systems (concentration, composition) means rapid methods are required for its characterization so that a high number of samples can be analyzed. The objective of the present study was to quantify the effects of spectrometer type and dissolved organic carbon (DOC) concentration on the humification indices of water-soluble fulvic acids (FAs) derived from synchronous fluorescence spectra, and thus enable the broader application of this method for DOM characterization. We used three standard FAs from the International Humic Substances Society, 24 water-soluble FAs isolated from topsoil, groundwater and surface water in a fen area, and two different spectrometers. The wavelengths at which bands occurred were similar for all the FAs. Therefore, the differences between the spectra of the FAs studied could be described by humification indices (band ratios). The humification indices calculated correlated very well between spectrometers despite small differences in the wavelengths of bands and shoulders. The absolute values of these indices deduced from two spectrometers can only be directly compared if the spectra are corrected using a standard substance. Increasing DOC concentration resulted in a linear increase in humification indices with a sample specific slope. Therefore, we recommend using an uniformly low DOC concentration of about 10 mg C l(-1) for recording the spectra of samples with typically low DOC concentrations (aquatic samples, soil solutions). This value is a compromise between relatively low absorption to minimize inner filter effects and a sufficient signal-to-noise ratio.
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