Development of MicroInterdigitated Electrode Arrays as Ion Sources for TIMS

在地质年代学和核取证等学科中，热电离质谱仪仍然是高精度测定元素同位素丰度的首选技术。然而，这种方法测量同位素的精确度取决于在仪器收集端测量的离子数量，而这本身取决于质谱仪中使用的技术的效率，即从天然和人造材料中提取的元素产生离子。对于铅和银等高电离势能金属，首选的电离方法是硅凝胶技术，该技术包括在感兴趣的金属中掺杂二氧化硅悬浮液，然后在质谱仪来源的金属带上将混合物干燥和加热到1300℃以上的温度。在这些温度下，二氧化硅混合物释放出稳定的金属离子流，在仪器中进行分析。这种电离方法效率低下，通常只产生约1%的电离。本项目的目标是提高这类离子源的效率。正在使用的方法基于这样一个事实，即硅凝胶技术实际上是一种液体玻璃离子发射器，它在质谱仪中高温蒸发过程中释放液体中产生的离子。因此，液体玻璃中产生的金属离子的比例可能会使用电解技术来增加，在这种技术中，液体玻璃在与两个相反电极性的金属电极(电化学电池)接触时作为电解液。电化学电池由一个微交叉指电极阵列(IDA)组成，该电极阵列是通过在未掺杂的硅或蓝宝石晶片上溅射钨而产生的。该设备正在通过一系列IDA原型组件进行开发，这些组件将包括IDA本身和一个高熔点温度陶瓷支架，该支架还将使IDA保持到位，以进行所需的电气连接，并允许从下方放置金属带作为加热器，使IDA达到工作温度(1200摄氏度至1300摄氏度)。各种原型配置旨在优化IDA内的加热和电气连续性，并最大限度地提高液态玻璃内产生的离子电流。一个主要目标将是将铅的电离效率提高至少五倍，以便提高对皮克大小的铅样品进行铀-铅年龄测定的精确度。第二个目标是开发一种用于私营部门热电离质谱仪的新型离子源并将其商业化。热电离质谱仪仍然是高精度同位素比值测定的金标准，也是目前提高U-Pb年龄测定精度的关键。同位素比值测定的精度受控于所计数的离子数目，而TIMS的一个主要限制是目前通过从受热金属带表面发射产生热化离子的方法电离效率低(~1%，很少~10%)。这项建议涉及开发一种新的离子源，它建立在“硅凝胶”技术的基础上，这是一种熔融硅酸盐液体离子源，于20世纪50年代末首次在TIMS中实施。最近的工作表明，大多数掺杂到熔融玻璃离子发射器中用于同位素分析的金属原子存在，并在蒸发过程中以中性原子而不是离子的形式释放，因此永远不会被送到仪器的分析器部分。该项目的目标是通过将熔体作为电化学池中的电解液来增加金属离子在这些熔体中的比例。这项工作的主要工作将是设计、制造和测试一种电化学池，其中微交指电极阵列(Micro-IDA)作为工作电极和对电极，并作为衬底，在其上沉积、熔化和电操纵金属掺杂硅酸盐以诱导金属掺杂的电离。在这种方法中，通过将两个电极的相对电势调节到从中性金属中移除一个电子所需的值来诱导金属掺杂的电离。然后，在熔融的硅酸盐蒸发过程中，金属离子被释放到质谱计的源头。小型微型IDA电极阵列非常适合这一目的，因为它们可以完全放置在质谱仪的焦平面上，其亚毫米大小的电极和电极间距应该保持被电解液润湿，即使在电解液蒸发并将金属离子释放到质谱仪中时也是如此，而且微型IDA很容易用传统的半导体制造技术根据用户的规格制造。该项目将涉及建立微型IDA设计、衬底和支架/加热器的组合，以在微型IDA表面有效熔化硅酸盐，并通过测量目前在科罗拉多博尔德大学运行的TIMS中的离子束强度和持续时间来确定铅的电离效率增加2-10倍。后一种仪器已经安装了为微型IDA供电所需的定制恒电位器。

Thermal ionization mass spectrometry remains the technique of choice for high precision determinations of the elemental isotopic abundances in such disciplines as geochronology and nuclear forensics. However, the precision of the isotopic measurements by this method depends on the number of ions measured at the collector end of the instrument, which is itself dependent on the efficiency of the techniques used in the mass spectrometer to generate ions from elements extracted from natural and manufactured material. The preferred ionization method for high ionization potential metals such as lead and silver is the Si-gel technique, which consist of doping a suspension of silica with the metal of interest and then drying and heating this mixture to temperatures over 1,300C on a metal ribbon in the source of the mass spectrometer. At these temperatures, the silica mixture emits a steady stream of metal ions which is analyzed in the instrument. This ionization method is inefficient, typically producing only about 1% ionization. The goal of this project is to improve the efficiency of this class of ion source. The approach being used builds on the fact that the Si-gel technique is actually a liquid glass ion emitter which releases ions generated in the liquid during high temperature evaporation in the mass spectrometer. As a result, the fraction of metals ions generated in the liquid glass likely can be increased using electrolysis techniques, in which the liquid glass serves as electrolyte when placed in contact with two metal electrodes of opposite electrical polarity (an 'electrochemical cell'). The electrochemical cell consists of a micro-interdigitated electrode array (IDA) produced by sputtering tungsten onto an undoped Si or sapphire wafer. The device is being developed through a series of prototype IDA assemblies that will consist of IDA, itself, and a high melting temperature ceramic holder that will also hold IDA in place for required electrical connections and will allow the placement of a metal ribbon from below that will serve as a heater to bring the IDA to operating temperatures (1,200C to 1,300C). The various prototype configurations are designed to optimize heating and electrical continuity within the IDA and to maximize the ion currents created within the liquid glass. A main goal will be to improve the ionization efficiency of lead by at least a factor of five in order to improve the precision uranium-lead age determinations for picogram size lead samples. A secondary goal is to develop and commercialize a new class of ion source for use in private sector thermal ionization mass spectrometers.The thermal ionization mass spectrometer remains the gold standard for high precision isotopic ratio determinations and is the key to current efforts to improve the precision of U-Pb age determinations. The precision of isotope ratio determinations is controlled by the number of ions counted and a major limitation in TIMS is that current methods of producing thermalized ions by emission from the surface of a resistively heated metal ribbon have low ionization efficiencies (~1%, rarely ~10%). This proposal involves the development of a new class of ion sources that builds on the 'Si-gel' technique, a molten silicate liquid ion source which was first implemented in TIMS in the late 1950s. More recent work has suggested that most metal atoms doped into a molten glass ion emitter for isotopic analyses are present and released during evaporation as neutral atoms, not ions, and so are never delivered to the analyzer portion of the instrument. The goal of this project is to increase the proportion of metal ions in these melts by treating the melt as an electrolyte in an electrochemical cell. The major effort in the proposed work will be the design, fabrication and testing of an electrochemical cell in which a micro-interdigitated electrode array (micro-IDA) serves as the working and counter electrodes and as the substrate upon which the metal doped silicate is deposited, melted and manipulated electrically to induce ionization of the metal dopant. In this method, the ionization of the metal dopant is induced by tuning the relative potential of the two electrodes to the value required to remove an electron from the neutral metal. The metal ions are then released to the source of the mass spectrometer during evaporation of the molten silicate. The small micro-IDA electrode arrays are ideal for this purpose because they can be wholly placed in the focal plane of the mass spectrometer, their submillimeter size electrodes and electrode spacing should remain wetted by electrolyte even as the electrolyte evaporates and releases metal ions into the mass spectrometer, and because micro-IDA are readily fabricated to user specifications with conventional semiconductor fabrication techniques. The project will involve establishing the combinations of micro-IDA design, substrate and holder/heater that produce effective melting of silicate on micro-IDA surface and produce a 2-10 fold increase in Pb ionization efficiency as determined by measurements of ion beam intensity and duration in the TIMS currently operating at the University of Colorado Boulder. The latter instrument is already fitted with the custom potentiostat required for powering the micro-IDA.