Effects of Li Isotopic Fractionation during Ion Exchange on the Measurement Accuracy of Li Isotopes
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摘要: 锂同位素被广泛应用于地球与行星科学各个领域，准确测定锂同位素比值是示踪各种自然过程的前提，但目前国际实验室报道的锂同位素标准物质测定值存在较大偏差，例如已报道的海水δ7Li测试值相差5‰。针对这一现状，本文基于离子交换理论基础，使用正态分布函数拟合淋出曲线，通过理论计算得到离子交换纯化过程造成的锂同位素分馏的理论值，该数值与MC-ICP-MS检测无关，但对锂同位素测试准确度有直接的影响。在此基础上，定义相对回收率（Rc）用于监测锂同位素分馏。基于本实验室分离纯化流程，通过理论计算得出，当Rc＞99.8%时，可认为离子交换纯化过程中没有引起可观察到的锂同位素分馏，进而不影响MC-ICP-MS检测准确度。目前世界上各实验室主要通过绝对回收率或Rc来判断分离过程中是否发生同位素分馏。由于测试的空间电荷效应，绝对回收率易被高估，而＞99%的Rc并未全部达到理论计算得到的Rc，表明各实验室对同种标准物质测试结果的偏差极可能是由于离子交换纯化过程中锂同位素分馏导致的。本文提出，对于每一样品，只需要分别测量离子交换过程中接收区间及其前后一定区间溶液中锂含量，将得到的Rc值与其理论值比较，即可判断分离纯化过程中是否引起可观察到的锂同位素分馏。Abstract:
BACKGROUNDLithium isotopes are widely used in various fields of earth and planetary science. Accurate determination of the lithium isotopic ratio is the premise of tracing various natural processes. However, there are large deviations in the measured values of lithium isotopic reference materials reported by international laboratories. For example, the reported seawater δ7Li test value differs by 5‰. To avoid this fractionation, laboratories employ their own empirical recovery values obtained from recovery tests rather than a theoretical one. OBJECTIVESTo investigate the reasons of this huge discrepancy and get an instructively theoretical value, so as to appraise Li fractionation during ion exchange and purification processes. METHODSBased on the theory of ion exchange, this study uses the normal distribution function to fit the leaching curve, and theoretically calculates the theoretical value of lithium isotope fractionation caused by the ion exchange purification process, which is independent of MC-ICP-MS detection. However, it has a direct impact on the accuracy of lithium isotope testing. On this basis, the relative recovery is defined to monitor lithium isotope fractionation. RESULTSThe fitting calculations show that only when recovery is higher than 99.8%, there would be no observable Li isotope fractionation caused by ion exchange and purification processes. At present, laboratories in the world mainly judge whether or not isotope fractionation occurs in the separation process by absolute recovery or relative recovery. Due to the space charge effect of the test, the absolute recovery is easily overestimated, and > 99% of relative recovery does not reach the theoretically calculated one, indicating that the deviation of the test results of the same reference material in each laboratory is most likely due to ion exchange. CONCLUSIONSIt is proposed that for each sample, it is only necessary to separately measure the lithium content in the receiving interval and the intervals before and after the ion exchange processes, and the obtained relative recovery value is compared with the theoretical value to determine whether observable lithium isotopic fractionation occurs during the separation and purification processes. -
Key words:
- Li isotopes /
- relative recovery /
- isotopic fractionation /
- MC-ICP-MS /
- theoretical calculation
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表 1 正态分布函数拟合不同溶液样品的Li淋出曲线对应参数
Table 1. Parameters of Li elution curves of various solute samples fitted by a normalized distribution model
样品 Li淋出曲线参数 μ(mL) σ A 拟合度(R2) 青海湖水首次过柱 39.79 1.39 206.29 0.96 青海湖水二次过柱 28.72 2.09 195.96 0.94 古土壤碳酸盐相 33.52 1.85 244.07 0.97 黄土硅酸岩相 33.51 2.73 280.94 0.97 黄土碳酸盐相 30.81 2.18 213.49 0.96 黄河河水 30.96 2.43 215.58 0.93 IEECAS-Li 36.36 2.89 307.10 0.98 注：表中A为Li淋出总量，拟合曲线来自苟龙飞等[11]。IEECAS-Li为本实验室工作溶液，详见苟龙飞等[11]。 
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