基于MC-ICP-MS的地质样品的高精度钾同位素分析方法

High precision potassium isotope analysis of geological samples using MC-ICP-MS

近年来,得益于多接收电感耦合等离子体质谱(MC-ICP-MS)的快速发展,钾(K)同位素的分析精度得到显著提升,极大地促进了K同位素地球化学的发展,在示踪大陆风化、壳幔物质循环等方面已经展现出良好的应用前景。目前,样品分离提纯过程繁琐,耗时长,以及ArH^(+)对K的直接干扰导致的分析精度不足依旧是K同位素得到更广泛应用的最大阻碍。在使用盐酸、硝酸以及氢氟酸将样品彻底溶解后,利用装有约2.7 mL AG50W-X8(BioRadTM,200~400目)阳离子交换树脂的定制石英离子交换柱,以及0.5 mol/L硝酸作为淋洗液可以有效地将地质样品中的K与Na、Ti、Mg、Mn、Al、Ca等主要基体元素一次性分离开来,从而有效分离提纯常见地质样品中的K(高Cr样品除外)。在仪器分析方面,为达到最大程度降低测试过程中的ArH^(+)产率以及提高仪器测试的稳定状态,分别采用了高分辨模式、高分辨加连续采集模式以及低分辨下扣除ArH^(+)干扰模式进行测试,结果表明低分辨模式下测试成本较低,测试稳定时间最长且能达到与高分辨率测试相比拟的分析精度(实验室长期精度~0.08‰)。在此基础上测定了一套中国国家岩石标准物质的K同位素组成,得出的K同位素测量值可以为今后不同实验室间的数据对比给出参考。

In recent years,with the rapid development of multi-collector inductively coupled plasma-mass spectrometry(MC-ICP-MS),the analytical precision of potassium(K)isotopes has been greatly improved,which in turn has promoted their development as potential geochemical tracers in processes involving continental weathering,crust-mantle interaction,and so on.Until now,the cumbersome and time-consuming process of sample pretreatments and the poor analytical accuracy caused by argon hydrides interference are still the biggest obstacles to the wide application of K isotopes.After completely dissolving the samples with hydrochloric acid,nitric acid,and hydrofluoric acid,purification of K from major matrix was performed on a customized quartz ion exchange column packed with~2.7 mL of AG50W-X8 cation exchange resin(BioRad TM,200~400 mesh)using 0.5 mol/L HNO:as the eluent.This one-step procedure can effectively separate K from Li,Na,Ti,Mg,Mn,Al and Ca,except for Cr,in most of the geological samples.To find the appropriate method for high-precision stable K isotopic measurements,for minimizing the ArH^(+)yield and improving the stability of the instrument,three methods were compared here(the high-resolution mode,high-resolution mode with continuous acquisition measurement run mode,and the low-resolution mode with a method designed to eliminate ArH^(+)interference).The low-resolution mode has the advantages of low cost,high stability,and high precision(long-term precision at about 0.08%o for our laboratory).When used to measure the K isotopic composition of a series of international geological reference materials,the results are consistent with previous values within analytical precision,indicating the reliability of this method.The K isotopic composition of a set of Chinese national rock standard materials is determined here.The measured values of K isotopes can provide a reference for the data comparison between different laboratories in future.