Varying amounts of isotopic exchange among dual water hydrogen isotope exchange methods indicate different pools of exchangeable hydrogen

The hydrogen isotopic composition of organic compounds carries information relating to the isotopic composition of biosynthetic source water, as well as source-organism biochemistry. This has led to diverse applications in areas such as paleoclimatology, ecology, and criminal forensics. Yet, measurement poses a unique isotopic challenge because hydrogen bound to oxygen or nitrogen can exchange with ambient water or vapor, unlike the hydrogen that is bound to carbon. This creates a need to account for this so-called exchangeable hydrogen. In some cases, this can be done by permanent replacement via chemical derivatization, but this is often not convenient or even possible. This has led to the development of dual water equilibration methods in which the exchangeable hydrogen in a sample is equilibrated with water with a known isotopic composition in a controlled manner as the last step in sample preparation prior to measurement. Dual water equilibration methods have facilitated applications in a range of subdisciplines, especially for applications focused on plant carbohydrate-rich materials such as cellulose and bulk wood, and on keratin in animal migration and ecology.

 

The term “exchangeable hydrogen” has generally been used inconsistently in environmental applications. In some cases, the term is used to describe only the hydrogen that can freely exchange with ambient vapor at room temperature conditions, while in other cases the term directly refers to all hydrogen that is not covalently bound to carbon and can therefore theoretically undergo isotopic exchange. These two definitions are inconsistent with one another because in many biomolecules, such as cellulose and keratin, a large portion of the hydrogen that is not carbon-bound is engaged in hydrogen bonding and is important for the macromolecular structure of the material. This bridging hydrogen, although not carbon-bound, is more difficult to isotopically exchange, and has the potential to be excluded by some types of dual water equilibration approaches. As a consequence, the fraction of hydrogen that is measured as exchangeable varies between sample types and methodologies, resulting in different hydrogen isotope values.

 

In this study we compared hydrogen isotope values after dual water equilibrations on plant carbohydrates and animal keratins using two different analytical approaches, one of which targeted only the freely exchangeable hydrogen pool, and the other of which targeted the theoretically exchangeable hydrogen pool. For all sample types, we observed large differences in the calculated fraction of exchangeable hydrogen, with the freely exchangeable approach yielding exchange rates 10-15 % smaller than those from the theoretically exchangeable approach. The data also showed a greater range of hydrogen isotope values for the approach that achieved higher degrees of hydrogen exchange, suggesting that the range in bridging hydrogen isotope values among samples was lower than that of carbon-bound hydrogen. We suggest modification of the term “exchangeable” in dual water equilibration studies to indicate whether the freely or the potentially exchangeable hydrogen is being targeted, and therefore the extent to which the bridging hydrogen has been isotopically exchanged.