Since the initial discovery of the non-exponential mass fractionation (non-EMF) of Nd isotopes analysis in 2002, similar deviations from an EMF pattern have been reported for measurements of a number of isotope systems (e.g., Si, Ge, Sr, Sn, Ba, Yb, W, Os, Hg and Pb) with MC-ICP-MS. However, the previous controversial reports on the magnitude of the deviations from EMF suggest that instrumental mass bias behaviour of MC-ICP-MS is neither fully understood nor well-characterised. Consequently, the standard approach of using a mass dependent fractionation (MDF) correction model (e.g., exponential law) may lead to both inaccurate and imprecise results. In this study, we systematically characterise the instrumental mass fractionation of MC-ICP-MS using Nd isotope measurements carried out under different plasma conditions, quantified using the normalised argon index (NAI) as an estimate of plasma temperature. Our results indicate that the mass bias of MC-ICP-MS is not always a simple exponential function of mass but shows systematic deviations from an EMF behaviour, which are closely associated with decreased NAIs. As a result, the conventional exponential correction yields a Nd/Nd value of 0.512257 for the reference material BHVO-2 when the NAI is low, which is 722 ppm lower than the reported value of 0.512979. By tuning the plasma to higher NAIs (higher plasma temperatures), the deviations from the EMF array are systematically attenuated and the exponential correction is able to correct for the instrumental mass bias under high NAIs. In contrast, a regression correction model for Nd isotopes is developed to account for the observed mass fractionation behaviour that does not follow EMF under low NAIs, given that the regression correction relies on the observed loglinear fractionation of different isotope pairs and does not require both isotope ratios to undergo EMF. We expect that the analytical protocol and fundamental insights gained in this study are applicable to a wide range of other isotope measurements with MC-ICP-MS.

中文翻译：

---

使用钕同位素阐明 MC-ICP-MS 的仪器质量分馏


自 2002 年首次发现 Nd 同位素分析的非指数质量分馏（非 EMF）以来，许多同位素系统（例如 Si、Ge、Sr、Sn）的测量都报告了与 EMF 模式的类似偏差。 、Ba、Yb、W、Os、Hg 和 Pb），使用 MC-ICP-MS。然而，之前关于 EMF 偏差大小的有争议的报告表明，MC-ICP-MS 的仪器质量偏差行为既没有被完全理解，也没有得到很好的表征。因此，使用质量相关分馏（MDF）校正模型（例如指数定律）的标准方法可能会导致不准确且不精确的结果。在本研究中，我们使用在不同等离子体条件下进行的 Nd 同位素测量来系统地表征 MC-ICP-MS 的仪器质量分馏，并使用归一化氩指数 (NAI) 作为等离子体温度的估计值进行量化。我们的结果表明，MC-ICP-MS 的质量偏差并不总是质量的简单指数函数，而是显示出 EMF 行为的系统偏差，这与 NAI 的降低密切相关。因此，当 NAI 较低时，传统指数校正得出的参考材料 BHVO-2 的 Nd/Nd 值为 0.512257，比报告值 0.512979 低 722 ppm。通过将等离子体调整到更高的 NAI（更高的等离子体温度），EMF 阵列的偏差会系统地减弱，并且指数校正能够校正高 NAI 下的仪器质量偏差。 相比之下，开发了 Nd 同位素的回归校正模型，以解释在低 NAI 下观察到的不遵循 EMF 的质量分馏行为，因为回归校正依赖于不同同位素对的观察到的对数线性分馏，并且不需要两种同位素经历 EMF 的比率。我们期望本研究中获得的分析方案和基本见解适用于使用 MC-ICP-MS 进行的各种其他同位素测量。