Structural investigation of coal humic substances by selective isotopic exchange and high-resolution mass spectrometry

Here, we report the application of a selective liquid-phase hydrogen/deuterium exchange (HDX) coupled to ultra-high resolution FTICR MS for structural investigations of individual constituents of humic substances (HS) isolated from three coal samples of different geographical origin. Selectivity was achieved by conducting reactions in DCl or NaOD solutions for catalyzing HDX in aromatic ring and side-chain positions with enhanced C-H acidity, respectively. FTICR MS analysis showed a significant overlap of molecular compositions in the HS samples under study, with 2000 common formulae. Using HDX, we demonstrated that the determined common formulae are presented by different structural isomers. We found that aromatic compounds varied both in the substitution pattern and the number of aromatic protons. Depending on the sample, lignin components with the same molecular formulae were composed of coumaryl, coniferyl or sinapyl moieties. Enumeration of HDX series for the 800 most abundant compounds showed that the results of HDX agreed well with the model structures suggested for humic components occupying a van Krevelen plot. In addition, we explored chemical transformations, which could connect individual constituents of coal HS. These transformations included hydrolysis of a guaiacyl moiety and reduction of a catechol unit, which corresponds to the conversion of a coniferyl fragment into a coumaryl unit. The obtained results were supportive of the hypothesis of the reducing humification pathway suggested for lignin transformation in the environment. The conclusion was made that the molecular ensemble of coal HS is composed of individual constituents produced at different humification stages.