Accurate determination of ultra-trace rare earth elements by LA-ICP-MS/MS and its application to cassiterite for effective elimination of Gd and Tb false positive anomalies†

Rare earth elements (REEs) are widely used as important geochemical tracers in earth and planetary sciences. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique has been routinely used for the determination of REE concentrations in various minerals. Nevertheless, it remained challenging to determine ultra-low REE contents (down to ng g⁻¹). Cassiterite with ng level REE contents shows false positive Gd and Tb anomalies in various datasets obtained by LA-ICP-MS. Herein, a novel analytical protocol for the accurate determination of ultra-trace REEs by LA-ICP-MS/MS is developed using oxygen as a reaction gas in mass shift mode, which avoids analytical artifacts caused by polyatomic interferences. Its application to cassiterite effectively eliminates Gd and Tb false positive anomalies. Both laser and solution cassiterite results have been used to prove the robustness of our protocol. The accuracy and precision of our approach is better than 10%. Our method can greatly facilitate the analysis of other geological, archeological, and environmental materials with large amounts of tin in the matrix that disturbs the REE measurement.