Sequential ultrasonic extraction of a Chinese coal and characterization of nitrogen‐containing compounds in the extracts using high‐performance liquid chromatography with mass spectrometry

Dongming lignite was sequentially extracted with petroleum ether, carbon disulfide, methanol, acetone, and isometric carbon disulfide/acetone mixed solvent at room temperature to afford extracts 1–5, respectively. High‐performance liquid chromatography coupled with electrospray ionization time‐of‐flight mass spectrometry was used to separate and characterize heteroatomic species in the extracts at molecular level. Molecular mass of compounds in the extracts is mainly distributed from 300 to 800 u , and the relative abundance of compounds with molecular mass over 800 u in the carbon disulfide extract is 135 times of that in the petroleum ether extract. The acetone extract has the highest relative abundance for organonitrogen compounds. Double bond equivalence numbers of detected species indicate that most of the organonitrogen compounds contain N‐heterocyclic aromatic rings, including pyridine, quinoline and pyrrole. Some organonitrogen isomers in Dongming lignite were separated and identified by high‐performance liquid chromatography coupled with electrospray ionization time‐of‐flight mass spectrometry, and the corresponding structural information was proposed.