Determination of the Isotopic Composition of Indium by MC-ICP-MS Using an Improved Measurement Model for the Gravimetric Isotope Mixture Method

In this study, we report the first independent and primary
isotope
ratio measurement of indium by MC-ICP-MS, utilizing gravimetric isotope
mixtures of two near-pure enriched indium isotopes. Consistent with
previous findings, we observe that the traditional one-mixture-at-a-time
calibration approach can introduce a strong dependence of the resulting
isotope ratio correction factors on the composition of the mixtures.
Our analysis suggests that this is due to biases inherent in measuring
the isotopic composition of pure isotopic materials. To address this
issue, we propose an improved calibration strategy that omits the
use of isotope ratios measured in the two near-pure indium isotopes,
at the expense of measuring additional isotope mixtures. The revised
calibration approach yields an indium isotope ratio n(113In)/n(115In) = 0.044 655
± 0.000 009 (95% confidence level) for the high-purity indium
isotopic reference material (NRC HIIN-1). This result is in agreement
with our 2010 result (0.044 72 ± 0.000 19, 95% confidence level),
obtained using the regression method with NIST SRM978a silver as a
calibrator, while showing an improvement in measurement uncertainty
by an order of magnitude.

本研究报道了首例采用多接收电感耦合等离子体质谱（MC-ICP-MS），以两种近纯富集铟同位素通过重量法制备的同位素混合标样为介质，开展的铟同位素比值独立直接测量。
与既往研究结果一致，我们发现传统的单次单一混合校准方法会导致最终的同位素比值校正因子显著依赖于混合体系的组成。
我们的分析表明，该现象源于纯同位素材料同位素组成测量中固有的系统偏差。
为解决该问题，我们提出了一种改进的校准策略：无需使用两种近纯铟同位素的同位素比值测量结果，代价是需要额外制备并测量更多的同位素混合体系。
经修正的校准方法得到高纯度铟同位素参考物质（NRC HIIN-1）的铟同位素比值n(¹¹³In)/n(¹¹⁵In) = 0.044 655 ± 0.000 009（95%置信水平）。
该结果与我们2010年采用以NIST SRM978a银作为校准物的回归法得到的结果（0.044 72 ± 0.000 19，95%置信水平）相一致，同时将测量不确定度降低了一个数量级。