Selective enrichment of fractions in thermal dissolution of high-sulfur coal: Application of solid-phase microextraction technology and construction of ZnO/rGO coatings

Selective enrichment of trace heteroatom compounds from the complex matrix of high-sulfur coal remains a major analytical challenge. In this work, a synergistically engineered ZnO/rGO nanocomposite coating was designed for hollow fiber solid-phase microextraction (HF-SPME) to enable selective enrichment of sulfur- and nitrogen-containing aromatics from thermally dissolved low-rank Indian high-sulfur coals. The coating integrates π-π interaction capability from rGO with Lewis acid-base coordination between ZnO and heteroatoms, forming a dual-interaction adsorption mechanism. After thermal dissolution (300 °C) in toluene, acetone, and isopropanol, soluble fractions were enriched under optimized HF-SPME conditions (35 °C, 30 min extraction). GC/MS and Orbitrap MS analyses demonstrated pronounced selectivity toward polycyclic aromatic hydrocarbons, heteroatom-substituted aromatics, and organosulfur species, while aliphatic compounds were weakly enriched. High-resolution MS combined with in-source collision-activated dissociation revealed preferential capture of highly conjugated (high double bond equivalent) sulfur-containing structures that are difficult to detect using conventional bulk extraction or direct spectroscopic methods. Compared with traditional coal analysis approaches, the ZnO/rGO-HF-SPME strategy provides higher selectivity, improved enrichment efficiency, and enhanced detection of trace heteroatom species. This method offers a targeted analytical platform for elucidating heteroatom occurrence in high-sulfur coal and provides molecular-level insight essential for advanced desulfurization and clean coal conversion processes.