Detection of C₄₀ Oxygenated Compounds in Marine Sediments Based on UPLC-Orbitrap MS

Terpenoids are a class of naturally occurring lipids biosynthesized by organisms, characterized by their isoprenoid unit structure. Following the death of an organism, these chemically stable biomarkers become buried and preserved in geological sediments. The characterization of the molecular composition of terpenoids in geological formations can effectively reflect the composition of the maternal biological communities, thereby providing crucial evidence for the reconstructing of paleoecosystems and biogeochemical cycles. However, the detection of lipids in natural environments is challenged by matrix interference, particularly in marine sediments, where the presence of inorganic salts significantly hinder lipid detection. This study employed ultra-high performance liquid chromatography coupled with Orbitrap mass spectrometry (UPLC-Orbitrap MS) to detect terpenoids in marine sediments. By utilizing the separation capability of liquid chromatography for compounds with various polarities, the method enabled online desalting and rapid detection of 78 distinct C₄₀ oxygen-containing compounds in marine sediment samples. Separation was performed using a BEH C18 column (100 mm×2.1 mm, 1.7 μm). The mobile phase consisted of a 0.1% formic acid aqueous solution and a 0.1% formic acid solution in acetonitrile-methanol (1:1, V/V) for gradient elution. MS data were acquired under positive ion electrospray ionization (ESI⁺) mode, with targets ionized as M+H⁺. Salts and organic matters eluted at different retention times. Under conditions that minimize inorganic salt interference, the molecular compositions of the detected C₄₀ compounds were deduced by matching their molecular formulas to the primary mass spectrometry data. The ion with the highest relative abundance at m/z 599.4098 was selected for tandem mass spectrometry fragmentation using high-energy collisional dissociation (HCD), and the detected compound was identified as myxoxanthophyll. Based on the principles of natural product chemistry and corroborated by tandem mass spectrometry (MS/MS) analysis, these oxygenated compounds were inferred to predominantly comprise xanthophyll-type tetraterpenoids. Xanthophylls were a major subclass of carotenoid pigments, which themselves represented an important category of tetraterpenoid pigments widely distributed in photosynthetic organisms. This study provides the molecular composition of C₄₀ oxygen-containing compounds along a gradient extending from estuaries to marginal seas. This composition serves as a reflection of the primary productivity structure dominated by diatoms, green algae, and brown algae in the water body of the East China Sea. This method enables real-time removal of polar salts and achieves online desalting of saline organic matter, providing an effective technical approach for the detection of terpenoid compounds in complex geological environments.