<div>Rationale: The hydrogen isotopic composition of lipids (δ 2Hlipid) is widely used in food</div><div>science and as a proxy for past hydrological conditions. Determining the δ 2H values of large,</div><div>well-preserved triacylglycerides and other uniquely microbial lipids, such as glycerol dialkyl</div><div>glycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been </div><div>possible due to their size which prohibits analysis by traditional gas chromatography</div><div>pyrolysis isotope ratio mass spectrometry (GC-P-IRMS).</div><div>Methods: We determined the δ 2H values of large, polar molecules and applied high</div><div>temperature gas chromatography (GC) methods on a modified GC-P-IRMS system. The</div><div>methods were validated using authentic standards of large, functionalised molecules</div><div>(triacylglycerides, TAG), purified reference standards of GDGTs, and compared to δ 2H</div><div>values determined by elemental analyser pyrolysis isotope ratio mass spectrometry (EA-PIRMS); and subsequently applied to the analysis of GDGTs in a sample from a methane</div><div>seep and a Welsh peat.</div><div>Results: δ 2H values of TAGs agreed within error between different between GC-P-IRMS and</div><div>EA-P-IRMS, with GC-P-IRMS showing 3-5 ‰ precision for 10 ng H injected. Archaeal lipid</div><div>GDGTs with up to three cyclisations could be analysed: δ 2H values were not significantly</div><div>different between methods with standard deviations of 5 to 6 ‰. When environmental</div><div>samples were analysed, δ 38 2H values of isoGDGTs were 50 ‰ more negative than those of</div><div>terrestrial brGDGTs.</div><div>Conclusions: Our results indicate that the high temperature GC-P-IRMS (HTGC-P-IRMS)</div><div>method developed here is appropriate to determine the δ 2H values of TAGs, GDGT lipids</div><div>with up to two cyclisations, and potentially other high molecular weight compounds. The</div><div>methodology will widen the current analytical window for biomarker and alimentary light</div><div>stable isotope analyses. Moreover, our initial measurements suggest that bacterial and</div><div>archaeal GDGT δ 2H values can record environmental and ecological conditions</div>
Determination of the δ 2H Values of High Molecular Weight Lipids by High Temperature GC Coupled to Isotope Ratio Mass Spectrometry
