The interconversion of δ²H values of collagen between thermal conversion reactor configurations

Rationale: Different thermal conversion reactor packings result in distinct δ²H values in nitrogen-containing materials, such as bone collagen. An older ‘traditional’ glassy carbon packing method causes incomplete conversion of N-containing samples into H₂ gas, resulting in altered δ²H values compared with the complete conversion of hydrogen obtained with a chromium-packed reactor. Given that δ²H values from collagen are gaining importance in palaeoecological and archaeological studies, a determination of the relationship between δ²H values produced with a glassy-carbon-packed and a chromium-packed reactor is needed. Methods: We obtained δ²H values (normalized on the VSMOW-SLAP scale) from both glassy-carbon-packed (GP) and chromium-packed (Cr) reactor configurations from bone collagen (n = 231) from a variety of archaeological sites, using a High-Temperature Conversion Elemental Analyzer (TC/EA) coupled to a Delta Plus XP isotope ratio mass spectrometer. Results: δ²H values from both methods are linearly correlated (r² = 0.934) and yield the following interconversion equation, δ²H(Cr) = 1.054 δ²H(GP) + 11.6‰ (95% conf. slope 1.020–1.090, intercept 10.6–12.6), and a mean difference of δ²H(Cr) – δ²H(GP) = 10.1‰ (1 sd 5.2, 1 se 0.3, n = 231). Conclusions: We recommend adopting this interconversion between δ²H values produced with a glassy-carbon-packed and chromium-packed reactor for bone collagen only, with appropriate propagation of uncertainty.