Accurate and precise measurement of Ce isotope ratios by thermal ionization mass spectrometry (TIMS)
Accurate and precise measurement of Ce isotope ratios by thermal ionization mass spectrometry (TIMS)
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DOI：
10.1016/j.chemgeo.2017.11.010
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发表时间：
2018
期刊：
影响因子：
3.9
通讯作者：
M. Willig;A. Stracke
中科院分区：
文献类型：
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作者：
M. Willig;A. Stracke
La-Ce同位素系统很少被应用，主要是由于分析困难。质谱仪的最新技术改进已经克服了其中一些技术限制，但 Ce 同位素分析仍然面临相当大的分析挑战。这主要与次要同位素 136Ce 和 138Ce 相对于主要同位素 140Ce 和 142Ce 的丰度较低有关（136Ce = 0.19%，138Ce = 0.25%，140Ce = 88.45%，142Ce = 11.11%）。因此，需要在大动态范围内同时测量离子束，从而导致各个离子束的计数统计不确定性存在巨大差异。此外，140Ce的大丰度引入了大的140CeO离子束对136CeO和138CeO离子束的拖尾效应，这需要充分的校正。在这里，我们提出了一种用于分析硅酸盐样品中 CeO 同位素比率的化学纯化方案和高精度热电离质谱 (TIMS) 方法。评估了 TIMS 数据采集的不同质谱策略的优缺点，包括使用 1010、1011 和 1012Ω 放大器以及 140CeO 尾部校正的不同策略。结合不同的尾部和基线校正方法，针对不同的高峰和非高峰收集方案制定了优化方案。结果表明，只要针对所采用的收集方案和尾部校正方法充分优化峰值、非峰值（半质量）和基线信号的积分时间，不同的策略就会产生具有 20–40 ppm (2 S.D.) 相似精度的 Ce 同位素比率。与之前的研究不同，我们使用1010Ω放大器获得了140CeO，并确定了长期平均值140Ce/142Ce=7.94319±2。使用常见的136Ce/142Ce=0.01688进行质量分馏校正，国际岩石标准物质BCR-1的136Ce/138Ce，本研究的 BCR-2 和 BHVO-2 与最近报道的结果非常吻合，所有 Ce 同位素比率均相对于平均 Ames Ce 金属的常见 138Ce/136Ce = 1.337366 进行报告。此外，还提供了其他几种广泛使用的国际岩石参考材料（AGV-2、BE-N、BIR-1、DNC-1、W2A）的 Ce 同位素比，便于实验室间比较。
The La-Ce isotope system has been rarely applied, mostly due to analytical difficulties. Recent technical refinements of mass spectrometers have overcome some of these technical limitations, but Ce isotope analyses still face considerable analytical challenges. These are mainly related to the low abundance of the minor isotopes136Ce and138Ce relative to the main isotopes140Ce and142Ce (136Ce = 0.19%,138Ce = 0.25%,140Ce = 88.45%,142Ce = 11.11%). Hence simultaneous measurement of ion beams over a large dynamic range is required, resulting in large differences in count statistical uncertainty on the individual ion beams. In addition, the large abundance of140Ce introduces a tailing effect of the large140CeO ion beam onto the136CeO and138CeO ion beams, which requires adequate correction. Here, we present a chemical purification scheme and high-precision thermal ionization mass spectrometric (TIMS) method for analyzing CeO isotope ratios in silicate samples. The advantages and disadvantages of different mass spectrometric strategies for data acquisition by TIMS were evaluated, including the use of 1010, 1011, and 1012Ω amplifiers and different strategies for the140CeO tail correction. An optimization scheme was developed for different on-peak and off-peak collection schemes, in combination with different tail and baseline correction methods. It is shown that, as long as the integration times for the on-peak, off-peak (half-mass), and baseline signals are adequately optimized for the employed collection scheme and tail correction method, different strategies yield Ce isotope ratios with similar precision of 20–40 ppm (2 S.D.). In contrast to previous studies, we have acquired140CeO by using a 1010Ω amplifier, and have determined a long-term average140Ce/142Ce of = 7.94319 ± 2. Using a common136Ce/142Ce = 0.01688 for mass fractionation correction, the136Ce/138Ce of the international rock reference materials BCR-1, BCR-2, and BHVO-2 of this study agree well with those recently reported, when all Ce isotope ratios are reported relative to a common138Ce/136Ce = 1.337366 for the average Ames Ce metal. In addition, Ce isotope ratios for several other widely available international rock reference materials (AGV-2, BE-N, BIR-1, DNC-1, W2A) are presented, and facilitate easy inter-laboratory comparison.