The interconversion of δ² H values of collagen between thermal conversion reactor configurations

Different thermal conversion reactor packings result in distinct δ² H 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 δ² H values compared with the complete conversion of hydrogen obtained with a chromium-packed reactor. Given that δ² H values from collagen are gaining importance in palaeoecological and archaeological studies, a determination of the relationship between δ² H values produced with a glassy-carbon-packed and a chromium-packed reactor is needed.

We obtained δ² H 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.

δ² H values from both methods are linearly correlated (r²  = 0.934) and yield the following interconversion equation, δ² H(Cr) = 1.054 δ² H(GP) + 11.6‰ (95% conf. slope 1.020-1.090, intercept 10.6-12.6), and a mean difference of δ² H(Cr) - δ² H(GP) = 10.1‰ (1 sd 5.2, 1 se 0.3, n = 231).

We recommend adopting this interconversion between δ² H values produced with a glassy-carbon-packed and chromium-packed reactor for bone collagen only, with appropriate propagation of uncertainty.