Optimal double and triple spiking for high precision lead isotopic measurement
Optimal double and triple spiking for high precision lead isotopic measurement
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DOI：
10.1016/s0009-2541(98)00203-4
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发表时间：
1999-05-24
期刊：
影响因子：
3.9
通讯作者：
Galer, SJG
中科院分区：
文献类型：
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作者：
Galer, SJG
由于仪器质量辨别的变化，通过热电离质谱 (TIMS) 进行的传统铅同位素比测量目前仅限于类似于 0.1% (2 sigma) 的外部精度，其大小只能使用外部标准或重复测量来评估。相比之下，双加标技术的目标是通过直接确定每个样品的质量辨别因子 epsilon 来完全规避此分馏问题。在对添加了已知校准组合物的双加标的样品等分试样进行第二次测量后，可以获得未知因子。迄今为止，双尖峰技术的实际应用使用了混合 Pb-207-Pb-204 示踪剂，取得了相当大的成功，但似乎很少或根本没有尝试优化所用示踪剂的成分。最佳的示踪剂成分必须满足两个标准：（1）内部测量误差与后验误差（误差放大因子）的比率应尽可能小； (2) 传播误差不应受到第二次混合运行的尖峰/样本比率的强烈影响——后者很重要，因为通常不能提前严格控制样本大小。与示踪剂成分本身同等重要的是用于求解 epsilon 的三个质量平衡方程组的选择；从几何角度来说，这相当于在特定的 3D 同位素空间中定义一条混合线。在四个这样的 3D 同位素空间中检查了潜在的尖峰成分，对应于 Pb-208、Pb-207、Pb-206 和 Pb-204 依次是常见的情况，即三个正交轴上比率的分母同位素。每个同位素空间内都存在三种可能的双尖峰族。还考虑了由铅的三种稳定同位素（即所谓的“三尖峰”）组成的示踪剂，其中每个同位素空间有三个族。首先使用几何方法评估这些尖峰成分的典型常见铅成分，然后通过使用离子统计误差模型确定误差放大系数来进行细化。结果表明，先前研究中使用的 Pb-207-Pb-204 双尖峰（其中 epsilon 在 Pb-208、Pb-207、Pb-206/Pb-204 同位素空间中测定）是最不适合使用的一种。用于确定 epsilon 的最佳示踪剂组成似乎仅限于两个家族：第一个是 Pb-207-Pb-204-Pb-206 三尖峰家族，其中 Pb-207/Pb-204 大约为一，同位素空间以 Pb-206 作为分母同位素；第二个家族由 Pb-204-Pb-207 双尖峰组成，在 Pb-208、Pb-206、Pb-204/Pb-207 同位素空间中解决。这两个系列均产生后验误差放大系数，校正后的 Pb-206/Pb-204 比率低至类似于 1.2，并且对于除高度不足尖峰和尖峰过度的混合物组合物之外的所有混合物组合物均小于 2。 (C) 1999 Elsevier Science B.V. 保留所有权利。
Conventional lead isotope ratio measurements by thermal ionization mass spectrometry (TIMS) are currently limited to external precisions of similar to 0.1% (2 sigma) due to variations in instrumental mass discrimination, the magnitude of which can only be assessed using external standard or duplicate measurements. The goal of the double-spike technique, by contrast, is to circumvent this fractionation problem entirely by determining the mass discrimination factor epsilon directly for each sample. The unknown factor can be obtained after performing a second measurement on a sample aliquot to which a double spike of known, calibrated composition has been added. Practical applications of the double-spike technique up until now have used a mixed Pb-207-Pb-204 tracer with considerable success, but there appears to have been little or no attempt to optimize the composition of the tracer used. An optimal tracer composition must fulfil two criteria: (1) the ratio of the internal, measured errors to a posteriori errors-the error magnification factors-should be as small as possible; (2) the propagated errors should not be strongly influenced by the spike/sample ratio of the second, mixture run-the latter is important since sample size cannot, in general, be rigorously controlled in advance. Of equal importance to the tracer composition itself is the choice of the three mass-balance equation set used to solve for epsilon; geometrically, this is equivalent to defining a mixing line in a particular 3D isotope space. Potential spike compositions were examined in four such 3D isotope spaces, corresponding to cases where Pb-208,Pb-207,Pb-206 and Pb-204 in turn are common, denominator isotopes of the ratios on the three orthogonal axes. Within each isotope space there exist three possible double-spike families. Tracers composed of three stable isotopes of lead-so-called 'triple spikes'-are also considered, of which there are three families per isotope space. These spike compositions were evaluated for a typical common lead composition, at first using geometric methods, and then refined by determining error magnification factors using an ion statistical error model. It is shown that the Pb-207-Pb-204 double spike used in previous studies, where epsilon is determined in Pb-208,Pb-207,Pb-206/Pb-204 isotope space, is one of the least suitable to use. Optimal tracer compositions for determining epsilon appear to be restricted to two families: The first is a family of Pb-207-Pb-204-Pb-206 triple spikes with Pb-207/Pb-204 of around unity and the isotope space has Pb-206 as the denominator isotope; the second family consists of Pb-204-Pb-207 double spikes, solved in Pb-208,Pb-206,Pb-204/Pb-207 isotope space. Both of these families yield error magnification factors on a posteriori, corrected Pb-206/Pb-204 ratios as low as similar to 1.2, and are less than 2 for all but highly underspiked and overspiked mixture compositions. (C) 1999 Elsevier Science B.V. All rights reserved.