<p>Determining the double-bond positions of monounsaturated compounds in the alcohol fraction in seep carbonate</p>

The unsaturation patterns of molecular fossils are critical in distinguishing their biological precursors and diagenetic processes. However, questions regarding the determination of double-bond positions of unsaturated dialkyl glycerol ethers (DAGEs) in submarine hydrocarbon seep ecosystems remain unsolved. To address this problem, a protocol for dimethyl disulfide (DMDS) derivative analysis using gas chromatography (GC)-mass spectrometry was optimised. Herein, the double-bond positions of monounsaturated short-chain alcohols, monoalkyl glycerol ethers (MAGEs), and DAGEs in seep carbonates were analysed. Among these compounds, the double-bond positions of trimethylsilyl (TMS) derivatives of the monounsaturated DAGE-DMDS adducts were determined for the first time, with mass spectra characterized by molecular ions (M (+)(.)) and two major diagnostic ions (omega(+)& nbsp;and delta(+) ) cleaved at the double bonds. For both the MAGEs and DAGEs, the double-bond positions of the monounsaturated n-C 16:1-alkyl moieties were identified at omega 5 and omega 7. Compared to monounsaturated short-chain alcohols and MAGEs, both ionization efficiency and relative sensitivity for the TMS derivatives of DAGE-DMDS adducts were low as indicated by the high limit of detection at a signal-to-noise ratio of 3. In addition, the appropriate injection parameters and oven temperature program during GC analyses may be crucial in determining the double-bond positions within monounsaturated DAGEs. (C) 2022 Elsevier B.V. All rights reserved