REASSESSING CALIBRATION

Variations in the 87Sr/86Sr ratio in archaeological tooth enamel are regularly used to study past mobility. While removing enamel samples by micro-drilling and analysing the chemically prepared strontium using TIMS is both reliable and accurate, laser ablation-multi collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) has been proposed, and used to a limited degree and success, as an alternative method for the rapid in-situ analysis of strontium isotopes. However, some studies report the presence of molecular interferences on all the Sr masses, with additional interference in bio-apatites of CaPO+ or ArPO+ on mass 87. This is particularly apparent in low strontium samples, such as enamel, and which cannot be chemically removed during LA-MC-ICP-MS. Without adequate elimination of, and/or correction for these molecular interferences strontium isotope ratios will be artificially high. Human and terrestrial animal tooth enamel typically have very low strontium concentrations (c.50- 300ppm), which are considerably below the concentration required to swamp the effect of this interference (c.1900ppm and above). While tuning for low oxides and using a collision cell can limit the formation of certain molecular interferences, this rarely removes them entirely and calibration to a set of preferably matrix-matched reference materials may still be required. Here we present recent work using a calibration set of low [Sr] human tooth enamels previously characterised isotopically by TIMS and investigate whether geogenic apatites can be used, to accurately correct for remaining interferences on mass 87.

Author(s): Rogers, Bryony (Department of Archaeology, Durham University; NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham) - Pashley, Vanessa - Horstwood, Matt (NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham) - Nowell, Geoff (Department of Earth Sciences, Durham University) - Evans, Jane (NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham) - Montgomery, Janet (Department of Archaeology, Durham University)