Determination of the δ2H values of high molecular weight lipids by high temperature GC coupled to isotope ratio mass spectrometry

RATIONALE The hydrogen isotopic composition of lipids (δ2 Hlipid ) is widely used in food science and as a proxy for past hydrological conditions. Determining the δ2 H 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 δ2 H 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, TGs), purified standards of GDGTs. The results compared with δ2 H values determined by high temperature elemental analyser pyrolysis isotope ratio mass spectrometry (HTEA/P/IRMS), and subsequently applied to the analysis of GDGTs in a sample from a methane seep and a Welsh peat. RESULTS The δ2 H 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: the δ2 H values were not significantly different between methods with standard deviations of 5 to 6 ‰. When environmental samples were analysed, the δ2 H 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 δ2 H 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 δ2 H values can record environmental and ecological conditions.