Determination of the δ2 H values of high molecular weight lipids by high-temperature gas chromatography 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 were 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 HTGC/P/IRMS and HTEA/IRMS, with HTGC/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 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.

Measurement of compound-specific carbon isotope ratios (δ(13) C values) via direct injection of whole crude oil samples.

RATIONALE: Stable isotope analysis is a powerful tool in understanding the generation, history and correlation of hydrocarbons. Compound-specific δ(13) C measurements of oils allow detailed comparison of individual compound groupings; however, most studies of these sample materials separate and isolate individual fractions based on the chemistries of particular compound groups, potentially losing considerable valuable isotopic data. Even if all fractions are analyzed, this represents a large increase in the data-processing burden, effectively multiplying data evaluation time and effort by the number of fractions produced. Gas chromatography/isotope ratio mass spectrometry (GC/IRMS) of untreated, whole crude oils allows the immediate collection of a larger suite of valuable isotopic data for these studies. METHODS: Untreated ('neat', undiluted), whole crude oils were directly injected and measured on a GC/IRMS system, using split (40:1) injections and a 50 m HP-PONA column. The GC method, 97 min in duration, was designed to maximize baseline separation of target analyte peaks, while an additional oxygen flow was admitted into the combustion reactor to maximize the lifetime of the combustion chemicals. RESULTS: The method and setup utilized allow the measurement of a much greater range of the n-alkanes (n-C4 to n-C25+ ) than traditional methods, while also retaining important cycloalkane, aromatic and isoprenoid peaks within the same analysis. Carbon isotope (δ(13) C) evaluation of these additional compound classes reveals trends in maturity and origins which are not identifiable when exclusively assessing the traditional n-alkane package (>n-C12 ). CONCLUSIONS: The described setup and method open up new possibilities for assessing the origins and histories of crude oil samples. The data generated for the whole oil n-alkanes by this method is equivalent to that reported for isolated n-alkane studies, while also providing valuable additional data on many other important compounds. The end result of this method is a more complete assessment of the carbon isotopic composition of crude oils. Copyright © 2016 John Wiley & Sons, Ltd.