Impact of varying HCl concentrations on the δ15N values of human rib bones used for dietary reconstruction

Abstract

While the effect of HCl on bone collagen extraction and IRMS measurements have been addressed previously, the effects of the variation of HCl concentration on the measurement ofδ15N analysis have never been reported. To this effect, “chunks” of four right rib shafts from one individual of the skeletal collection of Sainte-Marie-de-Beauce Cemetery (Canada, 1748–1878) were separated in 15 subsamples: three unacidified aliquots of different grain sizes (0–0.125 mm, 0.125 to 1 mm and >1 mm) and four aliquots that underwent three different HCl treatments (1%, 2% or 5%). Each aliquot was analyzed in duplicate or triplicate for δ15N depending on material availability. The unacidified samples show the largest δ15N variation (7.5 to 9.2‰) and the mean δ15N increase with HCl acid concentration, from 9.0‰ vs. AIR (1%) to 9.4‰ (5%). Also, we observe variations in δ15N from one analytical sequence to another while controls remain invariant. Therefore, we conclude that powdered samples (0–1 mm grain size) have to be thoroughly washed prior to analysis to remove soil contamination in trabecular bone, and that increases in δ15N in correlation with HCl concentration is likely the result of protein hydrolysis extent from 2 to 5% HCl solutions. Thus, unacidified >1 mm and samples acidified with 1% HCl provide the most reliable %N, C:N and δ15N values. The analytical sequences of the same sample producing different δ15N values, while control standards are invariant, remain unexplained. Replication of this study on cortical bones, while monitoring the impact on amino acids, would help to better understand acid pre-treatment effects on δ15N analysis for diet reconstruction.