In situ 87Sr/86Sr LA-MC-ICPMS on biogenic apatites: a matrix-matched standard correction approach

Strontium isotope ratios are a strong tool to study ancient hominin and animal migrations, hence the increasing need to have a simple, fast and microdestructive analytical technique to obtain accurate and precise 87Sr/86Sr ratios of precious tooth enamel and bone tissue. The traditional analysis by the TIMS or MC-ICPMS tecniques requires sample dissolution; therefore, several LA-MC-ICPMS methods have been developed to prevent sample destruction, particularly for prehistoric human teeth. Instrumental calibration on human enamel is difficult because of the typical low-Sr concentration and analytical interferences. In fact, the methodology for data reduction of in situ Sr isotopes of biogenic apatite is largely debated in the literature [e.g. 1, 2]. While monoatomic interferences (Kr, Rb, REE2+) are routinely corrected, the correction of polyatomic interferences (CaCa, CaAr and 40Ca31P16O) are challenging. In particular, the CaPO molecule strongly interferes on mass 87, hindering the achievement of precise and accurate 87Sr/86Sr ratios. Following on the work of Horstwood et al. (2008), we developed a method based on the concurrent analyses of multiple matrix-matched standard materials. We show how the linear regression of 87Sr/86Sr accuracy vs. 1/88Sr of at least three standards allows correction of this interference. During each analytical session, we analyse our four in-house matrix-matched standards (a human tooth, a bovine tooth, a swine tooth and a shark tooth) covering a wide range of Sr concentrations (from c.a. 100 ppm of the human tooth to the 1000 ppm of the shark tooth). A daily CaPO model is then built to predict the expected accuracy of the analysis. This correction gives an external reproducibility to the 4th decimal digit (e.g. 2σ-human enamel = 0.00047; c.a. 100 ppm) and an accuracy between the 4th and the 5th decimal digit when applied to analyses with a laser spot sizes of 100μm and a linear dynamic ablation pattern. Monitoring of the CaPO molecule formation during analysis is also achieved by performing several high resolution mass scans.