Effectiveness of different pre-treatments in recovering pre-burial isotopic ratios of charred plants

Rationale: Isotopic analysis of archaeological charred plant remains offers useful archaeological information. However, adequate sample pre-treatment protocols may be necessary to provide a contamination-free isotopic signal while limiting sample loss and achieving a high throughput. Under these constraints, research was undertaken to compare the performance of different pre-treatment protocols. Methods: Charred archaeological plant material was selected for isotopic analysis (δ¹³C and δ¹⁵N values) by isotope ratio mass spectrometry from a variety of plant species, time periods and soil conditions. Preservation conditions and the effectiveness of cleaning protocols were assessed through Fourier transform infrared spectroscopy and X-ray fluorescence (XRF) spectrometry. An acid–base–acid protocol, successfully employed in radiocarbon dating, was used to define a contamination-free isotopic reference. Acid–base–acid isotopic measurements were compared with those obtained from untreated material and an acid-only protocol. Results: The isotopic signals of untreated material and the acid-only protocol typically did not differ more than 1‰ from those of the acid–base–acid reference. There were no significant isotopic offsets between acid–base–acid and acid-only or untreated samples. Sample losses in the acid–base–acid protocol were on average 50 ± 17% (maximum = 98.4%). Elemental XRF measurements showed promising results in the detection of more contaminated samples albeit with a high rate of false positives. Conclusions: For the large range of preservation conditions described in the study, untreated charred plant samples, water cleaned of sediments, provide reliable stable isotope ratios of carbon and nitrogen. The use of pre-treatments may be necessary under different preservation conditions or more conservative measurement uncertainties should be reported.