Isotope Dilution Analysis for Particle Mass Determination Using Single-Particle Inductively Coupled Plasma Time-of-Flight Mass Spectrometry: Application to Size Determination of Silver Nanoparticles

Abstract: This paper describes methodology based on the application of isotope dilution (ID) in single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-ToFMS) mode for the mass determination (and sizing) of silver nanoparticles (AgNPs). For this purpose, and considering that the analytical signal in spICP-MS shows a transient nature, an isotope dilution equation used for online work was adapted and used for the mass determination of individual NPs. The method proposed measures NP isotope ratios in a particle-to-particle approach, which allows for the characterization of NP mass (and size) distributions and not only the mean size of the distribution. For the best results to be obtained, our method development (undertaken through the analysis of the reference material NIST RM 8017) included the optimization of the working conditions for the best precision and accuracy in isotope ratios of individual NPs, which had been only reported to date with multicollector instruments. It is shown that the precision of the measurement of these ratios is limited by the magnitude of the signals obtained for each NP in the mass analyzer (counting statistics). However, the uncertainty obtained for the sizing of NPs in this approach can be improved by careful method optimization, where the most important parameters are shown to be the selection of the spike isotopic composition and concentration. Although only AgNPs were targeted in this study, the method presented, with the corresponding adaptations, could be applied to NPs of any other composition that include an element with different naturally available isotopes.摘要：本文报道了一种基于同位素稀释（isotope dilution, ID）技术与单颗粒电感耦合等离子体飞行时间质谱（single-particle inductively coupled plasma time-of-flight mass spectrometry, spICP-ToFMS）模式联用的分析方法，用于银纳米颗粒（silver nanoparticles, AgNPs）的质量定量与粒径表征。考虑到单颗粒电感耦合等离子体质谱（single-particle inductively coupled plasma mass spectrometry, spICP-MS）的分析信号具有瞬态特性，本文适配了适用于在线分析场景的同位素稀释定量方程，用于单颗粒纳米颗粒的质量测定。所提方法采用颗粒级分析模式测定纳米颗粒的同位素比值，由此可实现纳米颗粒质量与粒径分布的完整表征，而非仅获取粒径分布的平均值。为获得最优分析性能，本研究通过参考物质NIST RM 8017的分析开展方法开发，优化了可实现单颗粒同位素比值最佳精密度与准确度的实验条件——截至目前，此类优化仅在多接收器质谱仪器中被报道过。研究表明，单颗粒同位素比值测定的精密度受限于质谱仪中单个纳米颗粒产生的信号强度（计数统计效应）。不过，通过严谨的方法优化可降低该方法用于纳米颗粒粒径测定时的不确定度，其中核心影响参数包括富集同位素的组成与浓度选择。尽管本研究仅针对银纳米颗粒展开实验，但经适当适配后，本文提出的方法可推广至其他含有天然同位素差异元素的纳米颗粒的分析。