磷灰石Sr同位素LA-MC-ICP-MS分析技术研究进展

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

  • 摘要: 磷灰石Rb/Sr比值极低,其Sr同位素组成可代表其寄主样品的初始Sr同位素组成,因此,被广泛应用于研究岩浆演化过程,示踪成矿流体的来源与演化,以及重建动物栖息环境和迁移历史。当前的LA-MC-ICP-MS可以使用60 ~ 120 μm激光束斑,对Sr含量大于500 μg/g磷灰石开展高精度的Sr同位素分析。随着磷灰石Sr同位素示踪应用领域的扩展,Sr同位素LA-MC-ICP-MS分析对象逐渐延伸到低Sr含量(如古人类牙齿、骨骼等),富稀土(如稀土矿床中的磷灰石),微米尺寸(如锆石中磷灰石包裹体,~10 μm)的磷灰石样品,这对该技术的抗干扰能力(尤其是抗40Ca31P16O+和REE2+干扰能力)和空间分辨率提出更高要求。本文重点评述了磷灰石Sr同位素LA-MC-ICP-MS分析过程中84,86Kr+40Ca31P16O+和REE2+等干扰的校正/消除方法的研究进展,以及各种提高其空间分辨率的技术和应用限制。通过优化ICP参数和改进MC-ICP-MS接口设计,降低MC-ICP-MS氧化物产率,可抑制40Ca31P16O+干扰对Sr同位素分析的影响;对于稀土二价离子干扰,建议采用改进的法拉第杯结构,并结合干扰信号扣除法予以校正;高阻值放大器和气溶胶快速导入系统都可以显著提高LA-MC-ICP-MS的灵敏度,应用于磷灰石Sr同位素分析可以提高空间分辨率。近年来出现的碰撞/反应池(CRC)多接收电感耦合等离子体串联质谱(CRC-MC-ICP-MS/MS)为磷灰石Sr同位素的干扰问题提供了全新解决方案,未来建议进一步开展基于LA-CRC-MC-ICP-MS/MS的磷灰石Sr同位素分析技术研究。

     

    Abstract: 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.

     

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