Method Development for LA-MC-ICP-MS Analysis of Sr Isotopes in Geological Samples

Strontium (Sr) isotopes are important geochemical tracers for investigating geological processes such as diagenesis, mineralization, and the sources of geological materials. Conventional bulk analytical methods have limited spatial resolution, making it difficult to resolve fine-scale isotopic variations within individual minerals. Although laser ablation (LA) in situ analysis provides high spatial resolution, it is challenged by multiple isobaric and polyatomic interferences (including Kr, REE²⁺, Rb, and Ca-related polyatomic species) as well as the complexity of interference correction. Therefore, this study established an in situ Sr isotope analytical method based on a laser ablation system coupled with a Nu Plasma Ⅱ multi-collector inductively coupled plasma-mass spectrometer (LA-MC-ICP-MS) and applied it to apatite, carbonate, and silicate samples. First, Sr isotope standard solutions doped with interfering elements were analyzed using solution nebulization MC-ICP-MS, and real-time simultaneous correction of Sr isotopes and interfering ion signals was performed. The interference evaluation demonstrated that Kr and REE²⁺ interferences could be effectively removed through mathematical correction, yielding corrected Sr isotope ratios consistent with the reference values within analytical uncertainty. The influence of Ca-related polyatomic interferences on Sr isotope measurements was found to be negligible, and the obtained isotope ratios remained within the uncertainty of the reference values even without correcting for these interferences. In contrast, ⁸⁷Rb was identified as the dominant source of interference, and maintaining a Rb/Sr ratio below 0.1 is critical for achieving high analytical accuracy. In addition, the laser operating conditions were optimized, and the optimal parameters were determined to be an energy density of 8–12 J/cm², a repetition rate of 10–12 Hz, and a spot size of 90–120 μm. The established LA-MC-ICP-MS method was subsequently applied to determine the Sr isotope compositions of apatite reference materials (Durango, MAD, and OtterLake) and carbonate reference materials (TARIM and JCt-NP). The measured ⁸⁷Sr/⁸⁶Sr ratios were 0.70635 ± 18 (2SD, n = 35), 0.71187 ± 7 (2SD, n = 36), 0.70426 ± 10 (2SD, n = 30), 0.71043 ± 6 (2SD, n = 20), and 0.70916 ± 3 (2SD, n = 11), respectively, all of which agree with the published values within analytical uncertainty. The analytical precision was better than 0.25‰ (2SD), demonstrating that the proposed method is capable of high-precision in situ Sr isotope analysis of apatite and carbonate samples at the microscale. However, the method exhibited relatively poor analytical accuracy for silicate samples with low Sr contents. Therefore, the present method is applicable to samples with Sr concentrations greater than 400 μg/g.