Re-evaluation of the hydrogen stable isotopic composition of keratin calibration standards for wildlife and forensic science applications

RATIONALE: Determination of non-exchangeable hydrogen isotopic compositions (delta H-2 values) of bulk complex organic materials is difficult due to uncontrolled H isotope exchange between the organic material and ambient water vapor. A number of calibration keratinous materials with carefully measured hydrogen isotopic compositions of the non-exchangeable fraction were proposed to enable stable isotope laboratories to normalize their H-2 measurements. However, it was recently reported that high-temperature carbon-reactor methods for measuring the hydrogen isotopic composition of nitrogenous organic materials is biased by the production of HCN in the reactor. As a result, the reported values of these calibration materials needed to be re-evaluated. METHODS: We evaluated the non-exchangeable delta H-2(VSMOW) values of keratins EC1 (CBS) and EC2 (KHS), USGS hair standards, and a range of other nitrogenous widely used organic laboratory calibration materials (collagen and chitin) using pre-treatment with a preparation device designed to eliminate residual moisture and quantify exchangeable H. RESULTS: The revised non-exchangeable d2HVSMOW values of EC-1 (CBS) and EC-2 (KHS) keratin standard materials were -157.0 +/- 0.9 and -35.3 +/- 1.1%, respectively. The revised values of USGS42 and USGS43 were -72.2 +/- 0.9 and -44.2 +/- 1.0%, respectively, in excellent agreement with previous results. CONCLUSIONS: For routine H isotope analyses, with proper sample pre-treatment, we show that the Comparative Equilibration approach can provide accurate andreproducible non-exchangeable delta H-2 values among laboratories regardless of the reactor type used. (C) 2017 Her Majesty the Queen in Right of Canada Rapid Communications in Mass Spectrometry (C) 2017 John Wiley & Sons Ltd. Reproduced with the permission of the Environment and Climate Change Canada.