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Abstract
Rationale: The hydrogen isotopic composition of lipids (δ 2Hlipid) is widely used in food
science and as a proxy for past hydrological conditions. Determining the δ 2H values of large,
well-preserved triacylglycerides and other uniquely 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 size which prohibits analysis by traditional gas chromatography
pyrolysis isotope ratio mass spectrometry (GC-P-IRMS).
Methods: We determined the δ 2H values of large, polar molecules and applied high
temperature gas chromatography (GC) methods on a modified GC-P-IRMS system. The
methods were validated using authentic standards of large, functionalised molecules
(triacylglycerides, TAG), purified reference standards of GDGTs, and compared to δ 2H
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
seep and a Welsh peat.
Results: δ 2H values of TAGs agreed within error between different between GC-P-IRMS and
EA-P-IRMS, with GC-P-IRMS showing 3-5 ‰ precision for 10 ng H injected. Archaeal lipid
GDGTs with up to three cyclisations could be analysed: δ 2H values were not significantly
different between methods with standard deviations of 5 to 6 ‰. When environmental
samples were analysed, δ 38 2H 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 δ 2H values of TAGs, GDGT lipids
with up to two cyclisations, and potentially other high molecular weight compounds. The
methodology will widen the current analytical window for biomarker and alimentary light
stable isotope analyses. Moreover, our initial measurements suggest that bacterial and
archaeal GDGT δ 2H values can record environmental and ecological conditions
