Improving the utility of LA-ICP-MS for isotope ratio environmental science

新的分析能力能够分析更少量的材料，直接导致地球和环境科学研究的新途径的发展。原位技术，如激光烧蚀电感耦合等离子体质谱法（LA-ICP-MS）和二次离子体质谱法（SIMS）允许直接分析少量材料（c.1-5ng），而不需要处理试剂的空白贡献，这限制了传统的基于溶解的方法。然而，对于小尺寸和/或低浓度分析物的材料，信噪比（SNR）限制了分析的精度。这就要求对大量材料进行分析，以达到所需的精度，即使是对敏感的原位技术也是如此。我们打算开发突破性的激光烧蚀采集和数据处理方法，以常规实现更高的信噪比和提高精度。这些方法将适用于皮克-纳克数量的材料，这取决于应用。为了证明这种能力，我们的主要应用将是用于核法医调查的1微米氧化铀（UOx）颗粒的铀同位素表征。这是一项具有国际重要性的应用，迫切需要对国际监测行动中收集的单个微米级铀颗粒进行表征。核材料的同位素比率揭示了它们的加工、起源和用途的细节。核能（238U/235U = 137.88）和核武器（238U/235U c. 1）需要从天然成分（238U/235U = 137.88）中获得不同程度的富集，而自然界中只发生极小的分馏（c.0.13%）。富集过程也会产生同样不同的236U/238U比率。因此，铀同位素比值可以成功地确定污染物UOx颗粒的来源和最终来源。因此，挑战在于分析单个细颗粒的铀同位素比率。其他技术，如：α和伽马射线能谱法、裂变径迹法、传统热电离质谱法（TIMS）、SIMS和传统溶液多收集器（MC-）ICP-MS，要么太耗时和/或昂贵，要么精度和/或分辨率低，要么受到显著的潜在背景和空白水平限制或干扰。激光烧蚀MC-ICP-MS为所有需要分析少量分析物的应用提供了潜在的解决方案，但前提是开发了新的方法，例如这里描述的方法。该提案详细介绍了如何使用新的分析技术，如液体激光烧蚀（LASIL），微体积烧蚀电池和火炬技术，单脉冲采集和全信号集成（TSI）数据处理技术，实现1微米颗粒中同位素比率的精确量化。这些方法都提高了图像的信噪比和空间分辨率。LASIL结合了LA的采样优势和溶液模式分析的信噪比增强，并提供了令人兴奋的可能性，在单滴中进行烧蚀后的单头清理和材料积累（有足够的样品可用），以达到可测量的浓度。所有这些方法的适当组合有可能成功分析1微米颗粒，并显着提高LA-ICP-MS在环境科学中的空间分辨率和实用性。该学生将受益于世界级同位素地球科学实验室NIGL的培训，以及英国最大的分析科学中心之一、专门从事LA-ICP-MS科学的拉夫堡大学的整合。该学生将参加该大学分析化学和环境科学理学硕士课程的相关模块，并参加研究生院的可转移和专业技能培训课程，还将受益于年度报告和升学课程。

New analytical capabilities enabling the analysis of smaller amounts of material lead directly to the development of new avenues of research in earth and environmental science. In-situ techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) allow small amounts of material to be analysed (c.1-5ng) directly without the blank contribution from processing reagents which limit traditional dissolution-based methodologies. However, for materials of small size and/or low concentrations of analyte, the signal:noise ratio (SNR) limits the precision of the analysis. This dictates analysis of larger amounts of material to achieve the required precision, even for sensitive in-situ techniques. We intend to develop ground breaking laser ablation acquisition and data handling methods to routinely achieve higher SNR's and enhance precision. These methods will be applied to picogram-nanogram quantities of material, depending on the application. To demonstrate this capability, our primary application will be the uranium isotopic characterization of 1micron uranium oxide (UOx) particles for nuclear forensic investigations. This is an internationally important application with a pressing need to characterize individual micron-sized uranium particles collected during international monitoring operations. Isotope ratios from nuclear materials reveal details about their processing, origin, and purpose. Different degrees of enrichment from natural compositions (238U/235U = 137.88) are required for nuclear power (238U/235U to c.33) and nuclear weapons (238U/235U c. 1) whilst only minimal (c.0.13%) fractionation occurs in nature. Enrichment processes also produce equally disparate 236U/238U ratios. Uranium isotope ratios can therefore successfully fingerprint the source and ultimate origins of contaminant UOx particles. The challenge is therefore to analyse uranium isotope ratios from individual fine particles. Other techniques such as: alpha and gamma-ray spectrometry, fission-track, conventional thermal ionisation mass spectrometry (TIMS), SIMS and conventional solution multi-collector(MC-)ICP-MS, are either too time consuming and/or expensive, have low precision and/or resolution, or suffer from significant potential background and blank level limitations or interferences. Laser ablation MC-ICP-MS offers a potential solution for all applications requiring the analysis of low amounts of analyte, but only if new methodologies, such as those described here are developed. This proposal details how accurate quantification of isotope ratios in 1micron particles will be achieved using new analytical techniques such as laser ablation in liquid (LASIL), micro-volume ablation cell and torch technology, single pulse acquisition and total signal integration (TSI) data processing techniques. All these methods enhance SNR's and improve spatial resolution in mapping. LASIL combines the sampling benefits of LA with the SNR enhancement of solution mode analysis and offers the exciting possibility of in-drop single-bead post-ablation clean up and accumulation of material (where adequate sample is available) in a single drop to achieve a measureable concentration. The appropriate combination of all these approaches has the potential to successfully analyse 1micron particles and dramatically improve the spatial resolution and utility of LA-ICP-MS applied to environmental sciences. The student will benefit from training at NIGL, a world-class isotope geoscience laboratory, and integration at Loughborough, one of the UK's largest analytical science centres and one specialized in LA-ICP-MS science. The student will attend relevant modules of the MSc programme in Analytical Chemistry and Environmental Science at the University and participate in the graduate school training programme in transferrable and professional skills, also benefiting from annual reporting and progression vivas.