Comparative high spatial resolution measurements of Sr isotopic composition in bio-apatite using different LA-MC-ICPMS configurations: application to faunal (sub)seasonal mobility studies†

The effects of isobaric interferences on the precision and accuracy of in situ Sr isotopic composition measurements in bio-apatite were investigated using different plasma conditions, interface configurations and instrumentation. Analysis of modern shark enameloid via laser ablation MC-ICPMS (LA-MC-ICPMS) Neptune and Neoma using H-type cones show similar long-term ⁸⁷Sr/⁸⁶Sr reproducibilities of 0.709194 ± 0.000034 and 0.709189 ± 0.000027 (2SD), respectively. Neither instrument showed detectable interference from matrix-generated ⁴⁰Ca³¹P¹⁶O or Ca dimer/argide isobars. The ratios agree with solution-based MC-ICPMS and TIMS analyses, yielding an average ⁸⁷Sr/⁸⁶Sr of 0.709176 ± 0.000008 (2SD). The use of a Jet sampler and a X-type skimmer cone with Neoma increases the sensitivity 2–3 times while maintaining the same level of accuracy. The formation of ⁴⁰Ca³¹P¹⁶O polyatomic clusters can be avoided by tuning the plasma to a low oxide level. An inherent feature of high-sensitivity cones is the production of Ca dimers/argides, which affects ⁸⁴Sr/⁸⁶Sr, additionally suffering from ⁶⁸Zn¹⁶O isobars readily generated at higher oxide levels owing to the ubiquitously high Zn concentration in bio-apatite. The mass bias of ⁸⁷Sr/⁸⁶Sr analysis follows the exponential law across all instrument configurations, which is interpreted as being due to a small average mass difference between normalised and normalising ratios. We demonstrate the benefit of high-sensitivity cones by analysing the highly compositionally zoned enamel of a woolly mammoth. Higher ion beam intensity allowed improved spatial resolution down to 40 μm diameter, revealing compositional variability obscured by larger spots. Improved counting statistics allowed for shorter integration times, resulting in <10 days of temporal resolution for >10 years of enamel formation. Increased sensitivity is of particular value for analysing organisms with low Sr uptake, such as humans, and in slowly mineralizing bioapatite tissues. Another benefit is the enhanced ability to distinguish early in vivo enamel formation from subsequent maturation, which is crucial for reconstructing the palaeoecology of large mammals and human life histories.