Unraveling the structural intricacy of synthetic humic substance via, UV-vis, FTIR, 13C NMR and non-targeted silylation-GC-Orbitrap MS

Abstract

Phenolic precursors combined with Maillard precursors can be converted into humic-acid like polycondensate during oxidation either via catalysis or non-catalysis processes. Current work was conducted on structural elucidation of synthetic humic substances (SHS) by avoiding alkaline extraction, using silylation coupled with high resolution mass spectrometery, while utilizing the mass spectral data of trimethylsilyl (TMS) derivatives. SHS mimicking natural humic substances was prepared by a bottom-up approach via polycondensation of catechol and Maillard precursors (glucose/glycine) at mild temperature (50 °C) and alkaline pH conditions. Reaction was monitored via UV–vis spectroscopy during the time period of 25 days by measuring the UV–vis curves. FTIR and 13C NMR curves were compared with IHSS Leonardite HA, gallic acid-protocatechuic acid-humic acid-like polycondensate (GA-PA-HALP), catechol: vanillic acid oligomer via oxidative coupling (CAVA) and corn straw HA via hydrothermal synthesis. SHS was subjected to silylation (BSTFA+1 %TMCS) in two solvents (pyridine or ethyl acetate) and then characterized via non-targeted analysis of GCOrbitrap MS. Mass spectrometery (MS) data showed that TMS fragments included more acidic and glucose oxidation moieties along with presence of Maillard reaction moieties. While remaining TMS fragments consisted of amines, aldehydes, ketones, amides, phenols etc. as well as conjugated moieties, adducts, quinones and oxidation/reduction moieties of catechol. Current method suggests that the combination of silylation with high-resolution gas chromatography-mass spectrometery (GCOrbitrap MS) offers an alternative approach for characterizing substances formed in biotic or abiotic reactions, as well as in water-extracted dissolved organic matter samples.