Determination of the δ 2H values of high molecular weight lipids by high-temperature gas chromatography coupled to isotope ratio mass spectrometry

Sabine Lengger*, Yuki Weber, Kyle Taylor, Robert Berstan, Ian D Bull, Jan Peter Mayser, William Leavitt, Jerome Blewett, Ann Pearson, Rich D Pancost

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

Rationale: The hydrogen isotopic composition of lipids (δ2Hlipid) is widely used in food science and as a proxy for past hydrological conditions. Determining the δ2H values of large, well-preserved triacylglycerides and other microbial lipids, such as glycerol dialkyl glycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been possible due to their low volatility which prohibits analysis by traditional gas chromatography pyrolysis isotope ratio mass spectrometry (GC/P/IRMS).

Methods: We determined the δ2H values of large, polar molecules and applied high temperature gas chromatography (HTGC) methods on a modified GC-/P/IRMS system. The system used a high temperature 7 m GC column, and a glass Y-splitter for low thermal mass. Methods were validated using authentic standards of large, functionalised molecules (triacylglycerides, TG), purified standards of GDGTs, and compared to δ2H values determined by elemental analyser pyrolysis isotope ratio mass spectrometry (HTEA/IRMS); and subsequently applied to the analysis of GDGTs in a sample from a methane seep and a Welsh peat.

Results: δ2H values of TGs agreed within error between GC/P/IRMS and HTEA/IRMS , with GC/P/IRMS showing larger errors. Archaeal lipid GDGTs with up to three cyclisations could be analysed: δ2H values were not significantly different between methods with standard deviations of 5 to 6 ‰. When environmental samples were analysed, δ2H values of isoGDGTs were 50 ‰ more negative than those of terrestrial brGDGTs.

Conclusions: Our results indicate that the high temperature GC/P/IRMS (HTGC/P/IRMS) method developed here is appropriate to determine the δ2H values of TGs, GDGTs with up to two cyclisations, and potentially other high molecular weight compounds. The methodology will widen the current analytical window for biomarker and food light stable isotope analyses. Moreover, our initial measurements suggest that bacterial and archaeal GDGT δ2H values can record environmental and ecological
conditions.
Original languageEnglish
Article numbere8983
Number of pages10
JournalRapid Communications in Mass Spectrometry
Volume35
Issue number4
Early online date17 Oct 2020
DOIs
Publication statusPublished - 28 Feb 2021

Keywords

  • Compound specific isotope analysis, Isotope ratio mass spectrometry, Hydrogen isotopes, Gas chromatography, High molecular weight compounds, High temperature gas chromatography

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