Triple measurement of δ15N, δ13C, and δ34S values in modern bone collagen at the DPAA Laboratory

Thuan H. Chau, Lesley A. Chesson, Gregory E. Berg

SNA International

Stable isotope ratio analysis of bone typically requires 1.0-1.5 mg of collagen for dual measurement of nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope delta values with an additional 10-15 mg required for separate measurement of sulfur (δ³⁴S). However, recent development of an EA-IRMS system with a ramped GC column allows for NCS measurement from a single 1.0 mg sample. Acquiring δ³⁴S values alongside δ¹⁵N and δ¹³C values is beneficial; thus, it is the analytical method the Defense POW/MIA Accounting Agency (DPAA) pursued for its isotope testing program. 

Employing a triple NCS method, we reanalyzed 371 modern human bone collagen samples that had been previously analyzed at an external provider via a dual NC method. The sample set included individuals from Asia (n = 288) and the Americas (n = 83). Isotope measurements were made using a Thermo Scientific™ EA IsoLink CN™ in line with a Delta V Plus™ mass spectrometer. Samples were weighed at 1.0 mg ± 5%. In each analytical “run,” five capsules of a QC material (Coll-MAR, a commercial collagen supplement) were interspersed between samples. 

Long-term precision of Coll-MAR (n = 85) was 0.14‰, 0.06‰, and 0.67‰ for δ¹⁵N, δ¹³C and δ³⁴S values, respectively. Paired t-tests indicated no significant differences for either δ¹⁵N or δ¹³C values between the external provider and the DPAA. The absolute mean difference (± SD) for δ¹⁵N and δ¹³C values was 0.16 ± 0.11‰ and 0.10 ± 0.08‰, respectively. Modern human bone collagen samples had a range of -6.11 to 16.00‰ for δ³⁴S values, which falls into ranges for other previously reported human populations.

This study demonstrated that the NCS isotope analysis method for bone collagen produces comparable δ¹⁵N and δ¹³C values to the NC method. The comparability of δ³⁴S values from the NCS method to a S-only method will requires additional large-scale investigation.