Advances in Strontium Isotope Analysis of Apatite Using LA-MC-ICP-MS: A Review

Apatite has an extremely low Rb/Sr ratio, and thus its Sr isotopic composition can represent the initial Sr isotopic composition of the host sample; therefore it is widely used to study magma evolution, trace the source and evolution of ore-forming fluids, and reconstruct the habitat environments and migration histories of animals. Using a 60–120 μm laser spot size, LA-MC-ICP-MS can perform high-precision Sr isotope analysis of apatite with Sr content ＞ 500 μg/g. However, as the application scope of apatite Sr isotope tracing expands, the analytical targets are being extended to apatite with low Sr content (e.g., ancient human teeth and bones), high rare-earth element content (e.g., apatite from REE deposits), and micron-scale dimensions (e.g., ～10 μm apatite inclusions in zircon), imposing higher demands on the technique’s interference resistance (especially against ⁴⁰Ca³¹P¹⁶O⁺ and REE²⁺ interferences) and spatial resolution. This review focuses on the progress in correcting/eliminating ^84,86Kr⁺, ⁴⁰Ca³¹P¹⁶O⁺, REE²⁺, and ⁸⁷Rb⁺ interferences in the Sr isotope analysis of apatite by LA-MC-ICP-MS, as well as various techniques for enhancing the spatial resolution of such analyses. By optimizing the ICP parameters to reduce the oxide yield of MC-ICP-MS, the influence of ⁴⁰Ca³¹P¹⁶O⁺ interference can be suppressed. For the interference of doubly charged rare-earth ions, an improved Faraday cup configuration combined with the peak-stripping method is recommended. Both high-resistance amplifiers and aerosol rapid introduction systems can significantly improve the sensitivity of MC-ICP-MS, and their application to Sr isotope analysis of apatite can enhance the spatial resolution. The recently emerged collision/reaction cell (CRC) multi-collector inductively coupled plasma tandem mass spectrometry (CRC-MC-ICP-MS/MS) offers a novel solution to the interference problems in apatite Sr isotope analysis. In the future, extensive research on apatite Sr isotope analysis based on LA-CRC-MC-ICP-MS/MS should be carried out.