高效分离Li及其同位素的MC-ICP-MS精确测定

Efficient purification for Li and high-precision and accuracy determination of Li isotopic compositions by MC-ICP-MS

为了高效地从地质/环境样品中分离纯化Li元素并进行Li同位素测定,经反复实验和改良发现:采用8 m L容积(树脂体积)离子交换柱,选取AG 50W-X12阳离子交换树脂,以0.5 mol/L HNO3为淋洗液,过柱一次,并收集20~48 m L区间的淋洗液,即可一步实现Li的完全纯化分离。对于高盐样品,建议过柱两次确保Na/Li<1,以达到上机测试的要求。由多种单元素标准混合的工作溶液(IEECAS-Li)经此流程分离后,采用Neptune Plus MC-ICP-MS测量得到的δ7Li值为8.31‰±0.12‰,与未混合的标准值(8.33‰±0.20‰)在误差范围内一致。采用此流程,获得的岩石标准物质AGV-2、BHVO-2和海水标准物质NASS-6的δ7Li值(2 s.d.,n=5)也与推荐值一致,分别为6.83‰±0.75‰、4.32‰±0.33‰和31.10‰±0.60‰。由此,我们建立了一套高效分离纯化Li及其同位素的MC-ICP-MS测试程序。将该程序用到Li含量在15 ng/g^90μg/g之间的实际样品中,δ7Li的长期内精度均好于0.30‰,且重现性高,表明该方法的分析精度和准确度都达到了国际标准水平。尤为重要的是,本方法可用于精确测量含痕量Li的环境样品的Li同位素组成。

Efficient isolation and purification of lithium （Li） were accomplished for various environmental and geological samples by a series of iterative experiments. These experiments led to the creation of a set of recommended experimental conditions including an 8 mL column volume, AG 50W-X12 resin, and 0.5 mol/L HNO3 eluent. Eluate was collected during a single leach in the volume interval between 20 and 48 mL to achieve a complete purification and recovery of Li. Nevertheless, for samples in a hyperhaline matrix, a double purification procedure is suggested to ensure that Na/Li 〈 1, which is essential for measuring the isotopic composition of Li on a multi-collector inductively coupled plasma mass spectrometer （MC-ICP-MS）. In this work, isotopic composition was determined using a Neptune Plus MC-ICP-MS. The 07Li of a laboratory-mixed solution （IEECAS-Li） purified by our procedure was measured to be 8.31‰ ± 0.12‰, which is in agreement with that of the corresponding single element standard before mixing, 8.33‰ ± 0.20‰. Following our purification procedure, the δ7Li values of USGS international rock reference materials （AGV-2 and BHVO-2） and seawater reference material NASS-6 were measured as 6.83‰ ± 0.75‰, 4.32‰ ± 0.33‰, and 31.10‰ ± 0.60‰ （2 s.d., n = 5）, respectively, all of which are within the range of error of the reported values. As a result, we propose a one-step purification procedure for Li and then an accurate and high-precision determination of its isotopic compositions by MC-ICP-MS. The purification protocol was then used to separate Li from samples of river water, lake water, and loess. The multiply measured δ7Li values of six such samples with Li concentrations ranging from 15 ng/g to 90 μg/g had two-time standard derivations better than 0.30‰ with excellent reproducibility, demonstrating the high-level precision and the accuracy possible for Li isotopic measurements. This purification protocol is especially suitable for environmental samples with very low Li contents.