Humification indices of water-soluble fulvic acids derived from synchronous fluorescence spectra - effects of spectrometer type and concentration

The high variability of dissolved organic matter (DOM) in natural systems ( concentration, composition) means rapid methods are required for its charac terization 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 d issolved organic carbon (DOC) concentration on the humification indices of water-soluble fulvic acids (FAs) derived from synchronous fluorescence spec tra, and thus enable the broader application of this method for DOM charact erization. We used three standard FAs from the International Humic Substanc es Society, 24 water-soluble FAs isolated from topsoil, groundwater and sur face water in a fen area, and two different spectrometers. The wavelengths at which bands occurred were similar for all the FAs. Therefore, the differ ences between the spectra of the FAs studied could be described by humifica tion indices (band ratios). The humification indices calculated correlated very well between spectrometers despite small differences in the wavelength s of bands and shoulders. The absolute values of these indices deduced from two spectrometers can only be directly compared if the spectra are correct ed using a standard substance. Increasing DOC concentration resulted in a l inear increase in humification indices with a sample specific slope. Theref ore, 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 concentr ations (aquatic samples, soil solutions). This value is a compromise betwee n relatively low absorption to minimize inner filter effects and a sufficie nt signal-to-noise ratio.