Spectral and mass spectrometric characteristics of different molecular weight fractions of dissolved organic matter

Dissolved organic matter (DOM) in water, especially the fluorescent DOM, has high reactivity with chlorine, resulting in toxic disinfection byproducts (DBPs). The effective removal of DOM before disinfection is a critical element to control the formation of toxic DBPs, which is closely related to the recognition of molecular structures for DOM, especially the fluorescent DOM. However, the limited information about the molecular features of fluorescent DOM left much room for the determination of DBP precursors. In this study, optical characteristics and molecular features of DOM fractions with varying molecular weight were illuminated by using fluorescence excitation-emission matrix spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Through Spearman's rank correlation analysis of fluorescent component and FTICR-MS peak relative intensities, 840 of formulae shared by different DOM fractions were associated with corresponding fluorescent components, accounting for 58.64% of FTICR-MS peak intensities of raw water. Most of these 840 molecular features are expected to directly account for the optical properties of aggregates for forming macromolecular fluorescent substances. Given the high chlorine reactivity of fluorescent compounds, these 840 molecular features shed some light on the unknown DBP precursors. Although the formulae associated with fluorescent components were illuminated, possible many isomers for a given formula made it complex to demonstrate the structure of DOM. Thus, further functional group characterization was performed via fourier transform infrared spectroscopy, which allowed a deep insight into the functional group structures of DOM.