IMPROVED ON-LINE MEASUREMENT OF NON-EXCHANGEABLE HYDROGEN IN BONE COLLAGEN AS FORENSIC ENVIRONMENTAL INDICATOR

The organic hydrogen isotope ratio (deuterium/hydrogen or D/H) in the tissues of terrestrial organisms is related to D/H values of precursor hydrogen in diet and water from precipitation. D/H in (fossil) bone collagen potentially characterizes the (paleo)environment of an animal. However, the original (paleo)environmental isotopic signal of organic hydrogen is largely limited to carbon-linked hydrogen (C-H). In contrast, organic hydrogen that is chemically linked to other elements (e.g., oxygen O-H and nitrogen N-H) is more loosely bound and exchanges with ambient water, even during storage and sample preparation. Further, the fraction of exchangeable hydrogen in total hydrogen can vary among samples. The uncertainty from exchangeable hydrogen can be reduced via equilibration in two waters with distinct isotopic compositions. Subsequent mass-balance calculations yield the D/H ratio of non-exchangeable hydrogen in collagen and the proportion of hydrogen that is exchangeable. Labor-intensive methods for isotopic equilibration of exchangeable organic hydrogen with water vapor have been used for more than 15 years. Here we present data from steam-equilibrated samples using a more efficient continuous-flow (i.e., on-line) approach using a ThermoFinnigan TC/EA fitted with a 50-position autosampler.

Collagens from bones of modern white-tailed deer (Odocoileus viginianus) and Southern mule deer (O. hemionus) across climate gradients in the USA were prepared for our preliminary study. Traditionally, TC/EA samples are wrapped tightly in impermeable silver capsules that limit the access of steam to collagen. Our samples were loaded into individual micro-perforated silver TC/EA cups. Cups were crimped shut and looked like small shopping bags. The perforations at the bottom of each cup were small enough that collagen could not spill out of the cup, but steam and gas could freely pass into and out of the cups. Steam equilibration of an entire carousel with up to 49 samples in an equilibration chamber occurred overnight with isotopically known steam at 115ºC, followed by drying with dry nitrogen, cooling, rapid transfer of the carousel to the TC/EA, and determination of D/H. We present preliminary results from collagens and discuss advantages of the new method.