Revealing the backbone structures of land-based fulvic acids derived from river and soil through n-butylsilane reduction.

While ultra-high-resolution mass spectrometry has enabled the identification of the molecular composition of dissolved organic matter (DOM), elucidating its molecular structure remains a challenging endeavor. Here, two fulvic acids (FAs), one from river and the other from forest soil, were subjected to reduction using an optimized n-butylsilane (n-BS) reduction method. The reduction products were purified through a combination of liquid-liquid extraction and silica gel column chromatography, resulting in the separation into saturates, aromatics, and polar products. The polar products were analyzed by high-resolution mass spectrometry (HRMS), and the saturates and aromatics were analyzed using gas chromatography-mass spectrometry (GC-MS). HRMS results showed that the number of oxygen atoms and double-bond equivalent (DBE) values of FA decreased after reduction. GC-MS results revealed that a total of 270 hydrocarbon monomers were identified from the reduction products of a single sample, with the highest carbon number of cycloalkanes reaching C₃₃. For the first time, steranes and hopanes were detected in the reduction products, potentially serving as evidence for the existence of carboxyl-rich alicyclic molecule (CRAM) precursors. Additionally, a significant number of polycyclic aromatic hydrocarbons were identified, and the potential sources of various compounds were preliminarily inferred based on their isomers. This study extends the knowledge of the possible backbone structure of the DOM and provides a new potential tool for investigating the origin and transformation mechanisms of DOM.