An improved protocol for 87Sr/86Sr by laser ablation multi-collector inductively coupled plasma mass spectrometry using oxide reduction and a customised plasma interface

Strontium isotope analysis has become one of the most effective means to characterise mobility and migration of humans and animals in the past. Analysis of strontium isotopes in bulk dental enamel by dissolution and chemical separation is precise, reliable and accurate. Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) offer high spatial, and therefore temporal, resolution sampling. However, the application of LA-MC-ICP-MS to strontium isotopes in bioapatites has been limited due to the presence of isobaric interferences. In particular, 40Ca31P16O+ (CaPO+) on 87Sr+ has been identified as causing a consistent positive offset in 87Sr/86Sr by LA-MC-ICP-MS, relative to dissolution methods, of approximately 500 to 1500 ppm. been demonstrated that tuning the mass spectrometer for reduced oxide production can reduce the size of this effect but not eliminate it completely. Here we present a revised protocol using a customised plasma interface through which helium gas is introduced. We find that using the oxide reduction and the customised plasma interface protocols can further reduce the offset such that we can achieve accuracy to within typical analytical precisions (± 50 ppm, 2σ) without any need for further mathematical corrections. We test the methodology by ‘blind’ comparisons between LA-MC-ICP-MS and micro-drill/dissolution thermal ionisation mass spectrometry on two archaeological teeth. Both techniques are in excellent agreement.