Sequential measurement of δ15N, δ13C and δ34S values in archaeological bone collagen at the Scottish Universities Environmental Research Centre (SUERC): A new analytical frontier

Rationale: The use of multi-isotopic analysis (delta N-15, delta C-13 and delta S-34 values) of archaeological bone collagen to assist in the interpretation of diet, movement and mobility of prehistoric populations is gradually increasing, yet many researchers have traditionally avoided investigating sulphur due to its very low concentrations (<0.3%) in mammalian collagen. For this reason, and as a consequence of analytical detection limits, sulphur is usually measured separately from carbon and nitrogen, which leads to longer analytical times and higher costs. Methods: A Thermo Scientific (TM) EA IsoLink (TM) isotope ratio mass spectrometry (IRMS) system, with the ability to rapidly heat a gas chromatography (GC) column and concentrate the sample gas online without cryo-trapping, was used at the Radiocarbon Laboratory at the Scottish Universities Environmental Research Centre (SUERC). Optimisation of the GC temperature and carrier gas flow rate in the elemental analyser resulted in improved signal-to-noise (S/N) ratio and sensitivity for SO2. This allowed for routine sequential N-2, CO2 and SO2 measurements on small samples of bone collagen. Results: Improvements in sample gas transfer to the mass spectrometer allows for sequential delta N-15, delta C-13 and delta S-34 values to be measured in 1-1.5 mg samples of bone collagen. Moreover, the sensitivity and S/N ratio of the sample gas, especially SO2, is improved, resulting in precisions of +/- 0.15 parts per thousand for delta N-15 values, +/- 0.1 parts per thousand for delta C-13 values and +/- 0.3 parts per thousand for delta S-34 values. Previous instrumentation allowed for the analysis of similar to 30 unknown samples before undertaking maintenance; however, similar to 150 unknown samples can now be measured, meaning a 5-fold increase in sample throughput. Conclusions: The ability to sequentially measure delta N-15, delta C-13 and delta S-34 values rapidly in archaeological bone collagen is an attractive option to researchers who want to build larger, more succinct datasets for their sites of interest, at a much-reduced analytical cost and without destroying larger quantities of archaeological material.