Advances in NCS bulk isotope analysis: getting more with less

Abstract:

The use of multi-isotopic analysis (δ¹³C, δ¹⁵N and δ³⁴S) of archaeological bone collagen to assist in the interpretation of diet, movement and mobility of prehistoric populations is increasing, yet many researchers have avoided analysing sulphur due to its very low concentrations in collagen. Although δ³⁴S values have proven to be an invaluable additional isotopic marker for ‘unpicking’ diet, the number of studies benefitting from its insight are low due to problems associated with producing, transferring and measuring SO₂ gas. In well preserved mammalian bone collagen, sulphur is present in concentrations of only ~0.2% to 0.3%, which has been a significant analytical challenge for sequential δ¹³C, δ¹⁵N and δ³⁴S analysis by EA-IRMS.

Previously, it was routine to analyse bone collagen samples twice; once for δ¹³C and δ¹⁵N (0.5–1 mg), and then a second time using a much larger sample (10–15 mg) for δ³⁴S. We demonstrate that the Thermo Scientific^TM EA IsoLink^TM IRMS system, which has the ability to rapidly heat a gas chromatography (GC) column and concentrate the sample gas online, during sample analysis, delivers sequential ¹³C, δ¹⁵N and δ³⁴S data from small samples of bone collagen (1–1.5 mg) from a single sample analysis. This means that the overall cost per sample analysis is reduced and less maintenance is required due to a significantly higher sample throughput, resulting in laboratory productivity increasing by 400%. Moreover, the sensitivity and signal-to-noise ratio of the sample gas, especially SO₂, has improved, resulting in routine precision's of ±0.1‰ for δ¹³C, ±0.15‰ for δ¹⁵N and ±0.3‰ for δ³⁴S. Importantly, critical issues linked with memory and oxygen isotope effects of sulphur are shown to be negligible, removing significant hurdles for routine δ³⁴S value determination. Whilst this example shows advances for bone collagen, the analytical benefits, however, extent to all sample matrices.

The ability to rapidly measure ¹³C, δ¹⁵N and δ³⁴S isotopic values sequentially in archaeological bone collagen is an attractive option to researchers that 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.