Study of the pH influence on the optical properties of dissolved organic matter using fluorescence excitation–emission matrix and parallel factor analysis

This study investigated the optical properties of dissolved organic matter (DOM) using the fluorescence excitation–emission matrix (EEM) method combined with parallel factor (PARAFAC) analysis. The examined DOM samples included Suwannee River fulvic acid (SRFA) and Nordic Reservoir natural organic matter (NRNOM). The optical properties of the DOM were determined from the behavior of individual groups of DOM fluorophores responding to different ionization states. The PARAFAC examination revealed that there were six independent EEM components in the SRFA that behaved differently in terms of pH influence on their fluorescence intensity. The dominant EEM peak locations (λex/λem) of these components were (230, 330) nm/420 nm (Component 1), (250, 350) nm/450 nm (Component 2), (240, 340) nm/460 nm (Component 3), (220, 280) nm/380 nm (Component 4), (270, 400) nm/490 nm (Component 5), and (280, 340) nm/540 nm (Component 6). However, only four EEM components were observed in the NRNOM, which are similar to Components 1, 2, 3 and 4 in the SRFA. It can be speculated that Components 1, 2, 3 and 6 can be categorized as traditional terrestrial humic-like chromophores, while they are composed of different fractions of carboxylic-like and phenolic-like chromophores; Component 4 can be categorized as protein-like fluorophores primarily composed of carboxylic-like chromophores, and Component 5 can be categorized as a microbial humic-like component that is overwhelmingly composed of phenolic-like fluorophores. The results demonstrated that PARAFAC analysis is a promising approach for characterizing the fine functions in DOM.