Quantitation of Interactions of Suwannee River Fulvic Acid with Protons Based on Numerical Deconvolution of Differential Absorbance and Fluorescence Spectra

Abstract

In this chapter, we applied an alternative approach that combined the generation of differential spectra with their deconvolution that allowed determining the emergence and significance of features seen in the spectroscopic data obtained at varying proton activities. The behavior of Suwannee River fulvic acid (SRFA) was ascertained using differential absorbance spectra and fluorescence excitation−emission matrixes. The pH-differential absorbance spectra were processed via numeric deconvolution to determine the contributions of discrete features. Deconvolution showed the presence of six Gaussian bands that were operationally referred to as A5, A4, A3, A2, A1, and A0 with maxima located in the order of increasing energy (or decreasing wavelengths) at 2.25 eV (551 nm), 3.24 ± 0.02 eV (383 nm), 3.99± 0.01 eV (311 nm), 4.52 ± 0.03 eV (275 nm), 5.11 ± 0.06 eV (243 nm), and 5.97 ± 0.03 eV (208 nm). The pH-differential excitation−emission matrixes of SRFA were processed by parallel factor analysis into six discrete fluorophores components referred to as F5, F4, F3, F2, F1, and F0 with maxima λ_ex/λ_em located at (220, 330) nm/430 nm, (250, 340) nm/450 nm, (240, 340) nm/460 nm, (220, 310) nm/380 nm, (270, 400) nm/490 nm, and (280, 340) nm/540 nm. Examination of relationships between the properties of the six Gaussian bands and the six fluorophores and their dependences vs. pH, as well as the comparison of the SRFA data and those for selected model compounds, allowed making conclusions concerning the engagement of potentially discrete functionalities (e.g., salicylic, polyhydroxyphenolic) and also of interactions characteristic of humic species only and indicative of their conformations.