Determination of Sr isotopes in calcium phosphates using laser ablation inductively coupled plasma mass spectrometry and their application to archaeological tooth enamel

When applied to an archaeological cattle tooth this approach shows Sr-isotope variations along the length of the tooth in agreement with independent TIMS data. The 40Ca-31P-16O polyatomic interference is the root cause of the bias at mass 87 during laser ablation ICP-MS analysis of inorganic and biogenic calcium phosphate (apatite) matrices. This results in inaccurate 87Sr/86Sr ratios even after correction of Ca dimers and doubly charged rare earth elements. This interference is essentially constant at specific ablation conditions and therefore the effect on 87Sr/86Sr data varies in proportion to changes in the Sr concentration of the ablated material. Complete elimination of this interference is unlikely through normal analytical mechanisms and therefore represents a limitation on the achievable accuracy of LA-(MC-)ICP-MS 87Sr/86Sr data without rigorous calibration to known reference materials.