An efficient method for isolating individual long-chain alkenones for compound-specific hydrogen isotope analysis

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Hydrogen isotope ratios (H-2/H or D/H) of long-chain unsaturated ketones (alkenones) preserved in lake and marine sediments hold great promise for paleoclimate studies. However, compound-specific hydrogen isotope analysis of individual alkenones has not been possible due to chromatographic coelution of alkenones with the same carbon chain length but different numbers of double bonds. Published studies have only reported the delta D values of the mixture of coeluting alkenones. We developed an efficient procedure to isolate individual alkenones based on double-bond numbers using silica gel impregnated with silver nitrate. The chromatographic procedure is simple, inexpensive, and highly reproducible, offers 87-100% sample recovery, and allows for the first time hydrogen isotopic measurement on individual alkenones. delta D values of specific di-, tri- and tetraunsaturated C-37 alkenones produced by an Emiliania huxleyi culture, as well as those isolated from Greenland lake sediments, differ consecutively by 43-65 %. These findings suggest that alkenones with different numbers of carbon-carbon double bonds express significantly different delta D values and that coelution of different alkenones may lead to erroneous source water delta D reconstructions. Our alkenone isolation approach opens a new avenue for paleoclimate reconstructions using hydrogen isotope ratios of individual alkenones.