Development of a Simplified Cavity Thermal Ionization Source for Geoscience Applications

用于确定材料化学和同位素组成的仪器的改进在从材料科学到医学的广泛领域取得了重大进展。通常，天然物质，如岩石和环境样品，被证明是最难分析的，因为它们是元素周期表中每种元素的复杂混合物。任何元素最显著的特征就是它的质量和同位素组成。这导致了20世纪20年代S质谱计的发明，并逐渐扩大了这种仪器的能力，因此现在每当需要进行化学分析，特别是同位素分析时，大量不同类型的质谱计就构成了最广泛使用的仪器。质谱仪的灵敏度取决于待分析元素(S)转化为带电离子的效率，即电离效率。该项目将在橡树岭国家实验室所做的基本仪器开发的基础上，扩大腔离子源在地球科学中用于化学和同位素测量的使用。腔式离子源由一个狭窄(直径1 mm)的管子组成，样品放入其中。然后将空腔加热到极端温度(高达3000摄氏度)，导致样品蒸发。单个样品原子在到达空腔开口的过程中，会反复反弹到空腔的壁上。当一个原子接触到腔体的热金属表面时，它有一个有限的电离概率。一旦发生这种情况，在空腔开口处施加的电场就会提取离子，并将其加速进入质谱仪。空腔离子源的电离效率比传统仪器--热电离质谱计--高出10-50倍。空腔离子源电离效率的提高将使分析范围扩大到比目前可能小10-50倍的样品大小，或者允许以更高的精度分析大样品的同位素组成。该项目将重点研究固体地球形成的过程和时间尺度，以及它如何以及何时分离成核、地幔和地壳，从而形成宜居环境，并将使生命得以生存和繁荣的资源集中在地球上。这笔赠款将允许开发一种简化的腔热离子源，这将极大地提高广泛用于地球科学研究的质谱仪的灵敏度。近一个世纪以来，热电离质谱(TIMS)一直是地球科学中的一种重要工具，涉及依赖于放射性年代学、同位素示踪和高灵敏度化学分析的广泛课题。腔式热离子源，如用于核实验室的热离子源，可以为各种在地球科学中有用的元素(例如，铬、锶、钕、铅)提供高达10-50倍的电离效率。这一量级的灵敏度提高转化为样本量的类似减小，以便以当前的精度水平进行同位素测量，或者允许对与当前使用的样本量类似的样品进行更高精度的同位素比率分析。该项目的目标是设计一种空腔离子源，它可以很容易地适应地学中常用的商用TIMS类型，以提供极大地增强的电离效率，同时保持现代TIMS仪器的简单性、清洁性和易用性。同位素比率测量解决了地球科学中的许多第一级问题，从为辐射地质年代学提供基础，到追踪许多影响地球表面和内部的主要化学过程。一种离子源可以提供比现有源高10倍或更多的灵敏度，但保持现有仪器的简单性、易用性和清洁度，将很快被地球科学界和服务于该社区的商业仪器制造商采用。该项目还将培训一名早期职业科学家，他们不仅将学习按计算机上的哪些按钮来指示仪器产生数据，还将学习质谱仪的基本物理知识。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Improvements in the instrumentation used to determine the chemical and isotopic composition of materials have provided critical advances across a wide variety of fields from material science to medicine. Often, natural materials, such as rocks and environmental samples, prove to be the most difficult to analyze as they are complex mixtures of every element in the periodic table. The most distinguishing characteristic of any element is its mass and isotopic composition. This gave rise to the invention of the mass spectrometer in the 1920's and to the gradual expansion of the capabilities of this type of instrument so that now a vast array of different types of mass spectrometers constitute the most widely used instrument whenever chemical, and particularly isotope, analyses are needed. The sensitivity of the mass spectrometer depends on how efficiently the element(s) to be analyzed can be turned into electrically charged ions, termed ionization efficiency. This project will build on basic instrumentation developments done at Oak Ridge National Laboratory to extend and expand the use of a cavity ion source for chemical and isotopic measurements applied to the geosciences. The cavity ion source consists of a narrow (1 mm diameter) tube into which the sample is placed. The cavity is then heated to extreme temperatures (up to 3000 C) causing the sample to evaporate. Individual sample atoms repeatedly bounce against the walls of the cavity on their way to the cavity opening. Whenever an atom touches the hot metal surface of the cavity, it has a finite probability of becoming ionized. Once it does, the electric field applied at the opening of the cavity extracts the ion and accelerates it into the mass spectrometer. Cavity ion sources have demonstrated ionization efficiencies 10-50 times greater than that of the conventional instruments, known as thermal ionization mass spectrometers. The improvement in ionization efficiency in cavity ion sources will allow analyses to be extended to sample sizes 10-50 times smaller that currently possible, or allow the isotopic composition of large samples to be analyzed to much higher precision. This projects application of the enhanced sensitivity of the cavity ion source will focus on study of the processes and timescales by which the solid Earth formed, and how and when it separated into core, mantle, and crust leading to the habitable environments and concentrating the resources that allow life to survive and prosper on Earth.This grant will allow development of a simplified cavity thermal ion source that will dramatically improve the sensitivity of a type of mass spectrometer widely used for geoscience studies. Thermal ionization mass spectrometry (TIMS) has been a critical tool in geosciences for almost a century, on a wide range of topics that depend on radioactive chronology, isotope tracing, and high-sensitivity chemical analyses. Cavity thermal ion sources, like those used in nuclear laboratories, can offer up to a factor of 10-50 increase in ionization efficiency over conventional TIMS for a variety elements that are useful in the geosciences (e.g., Cr, Sr, Nd, Pb). Sensitivity improvements of this magnitude translate to similar magnitude reductions in sample size to make isotopic measurements at current levels of precision, or alternatively allow much higher precision isotope ratio analyses for sample sizes similar to those used currently. The goal of the project is to design a cavity ion source that can be readily adapted to the type of commercial TIMS commonly used in the geosciences to provide greatly enhanced ionization efficiencies while retaining the simplicity, cleanliness, and ease of use of the modern TIMS instrument. Isotope ratio measurements address many first order questions in the geosciences, from providing the basis for radiometric geochronology, to tracing of many of the main chemical processes that act to modify Earth's surface and interior. An ion source that can offer factors of 10 or more increased sensitivity over existing sources, but retains the simplicity, ease of use, and cleanliness of existing instruments would find rapid adoption by the geoscience community and the commercial instrument manufacturers that serve that community. The project also will involve the training of an early career scientist who will learn not only which buttons on the computer to push to tell the instrument to produce data, but the basic physics of the mass spectrometer.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.