Simplified batch equilibration for D/H determination of non-exchangeable hydrogen in solid organic material

Abstract

Hydrogen isotopic analysis of organic materials has been widely applied in studies of paleoclimate, animal migration, forensics, food and flavor authentication, and the origin and diagenesis of organic matter. Hydrogen bound to carbon (C-H) generally retains isotopic information about the water present during organic matter synthesis and associated biosynthetic fractionations, but hydrogen bound to other elements (O, S, or N) can readily exchange with atmospheric water vapor and reflects recent exposure to water or vapor. These two pools must be separated to obtain meaningful information from isotope ratios of organic materials. Previously published analytical methods either replace exchangeable H chemically or control its isotopic composition, usually by equilibration with water or waters of known isotopic composition. In addition, the fraction of H that is exchangeable can vary among samples and is itself of scientific interest. Here we report an improved and automated double-equilibration approach. Samples are loaded in a 50-position autosampler carousel in an air-tight aluminum equilibration chamber. Water vapor of known isotopic composition is pumped through the chamber at 115°C for at least 6 h. After flushing with dry N_2 and being cooled, the carousel is rapidly transferred from the equilibration chamber to a He-purged autosampler attached to a pyrolysis elemental analyzer connected to an isotope ratio mass spectrometer. By equilibrating two aliquots of each sample with two isotopically distinct waters, it is possible to calculate both (1) the D/H ratio of non-exchangeable H, and (2) the fraction of H that is exchangeable. Relative to previous double-equilibration techniques, this approach offers significant reductions in sample size and labor by allowing simultaneous equilibration of several tens of samples.

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