Strontium isotope analysis of curved tooth enamel surfaces by laser-ablation multi-collector ICP-MS

Abstract

Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is increasingly applied to measure the strontium isotope composition (87Sr/86Sr) of fossil or modern tooth samples in order to address questions about mobility. Recently, concerns have been raised that an analytical bias due to instrumental isotopic mass fractionation might be introduced when this method is used to sample across naturally curved tooth enamel surfaces.We address this concern by reanalyzing data that were originally produced using 750μm linear LA-MC-ICP-MS scans on the external, slightly curved surfaces of fossil hominin teeth (Australopithecus and Paranthropus) from South Africa. By re-integrating the first and last 1/3 of the data along the original 750μm linear scans, we compared 87Sr/86Sr results produced by sections of each scan with varying degrees of laser focus. The results show no evidence of any analytical scatter between strontium isotope values for the different sections of the curved tooth surfaces and results for the different sections of each linear scan agree well within the precision of the method (external 2σ ± 0.0004). Furthermore, each analytical session was bracketed by analysis of a curved rodent (Otomys sp.) tooth in-house standard with up to ±100μm vertical change along the 750μm linear scan. Long-term, average LA-MC-ICP-MS 87Sr/86Sr results for one such Otomys tooth standard (0.72989±0.00029, n=71) agree well with the solution MC-ICP-MS analysis of the same tooth (0.72976±0.00002). New LA-MC-ICP-MS 87Sr/86Sr ratios, using both a 213nm and solid-state 193nm laser ablation system, for sets of consecutive 750μm linear scans along the curved, outer tooth enamel surface on another single Otomys tooth in-house standard agree within ±0.0004 regardless of progressively more extreme vertical change during analysis (±10μm to ±900μm).These results are similar to LA-MC-ICP-MS Sr isotope analyses, using the same instrumentation and methodology, of an in-house, polished, flat clinopyroxene mineral standard (LA: 0.70482±0.00029, n=155; solution: 0.70495±0.00003).We conclude that, provided the stable 86Sr/88Sr ratio is used in conjunction with the exponential law to constantly correct for instrumental isotopic mass fractionation during acquisition, curved tooth enamel surfaces represent no impediment to accurate LA-MC-ICP-MS strontium isotope analyses.