Procedure for the Classification of Fulvic Acids and Similar Substances Based on the Variation With pH of Their Synchronous Fluorescence Spectra

Abstract

A method for the classification of fulvic acids (FA) and similar substances is presented. It consists of three steps: (i) for each sample, sets of synchronous molecular fluorescence spectra as the pH is changed are acquired; (ii) the spectral set is reduced to a small number of spectra and concentration profiles of the detected components by evolving factor analysis with a gradient concentration window (EFA–GCW) are obtained; (iii) the spectra of the components of the samples are subjected to pattern recognition analysis by cluster analysis and multi-dimensional scaling. The procedure was tested with 13 samples of FA of natural and anthropogenic origin. Classification rules as a function of the origin and acid–base properties of the samples, in the form of figures with spectra and tables with spectral characteristics, were obtained. The classification according to the origin of the FA samples is useful for the evaluation of the extent of humification of FA. The following information on the acid–base properties of the fluorescent fractions of FA was obtained: only FA extracted from soil have fluorescent fractions with relatively strong acid character (pK_a ≈ 3.2); all FA extracted from different origins have similar fluorescent structures with relatively weak acid character (pK_a ≈ 6.2); in addition, FA extracted from low humified materials (leaf litter and composted wastes) have another set of similar fluorescent structures with weak acid character (pK_a ≈ 9.2).