Development of MicroInterdigitated Electrode Arrays as Ion Sources for TIMS

在地质年代学和核法医学等学科中，热电离质谱法仍然是高精度测定元素同位素丰度的首选技术。 然而，通过该方法的同位素测量的精度取决于在仪器的收集器端处测量的离子的数量，其本身取决于质谱仪中用于从从天然和人造材料提取的元素产生离子的技术的效率。 对于高电离电位金属如铅和银，优选的电离方法是硅凝胶技术，其包括用感兴趣的金属掺杂二氧化硅悬浮液，然后在质谱仪源中的金属带上干燥并加热该混合物至超过1,300 ℃的温度。 在这些温度下，二氧化硅混合物会释放出稳定的金属离子流，并在仪器中进行分析。这种电离方法效率低，通常仅产生约1%的电离。本项目的目标是提高这类离子源的效率。 所使用的方法建立在以下事实的基础上：Si-gel技术实际上是液体玻璃离子发射器，其在质谱仪中的高温蒸发期间释放液体中产生的离子。 因此，在液体玻璃中产生的金属离子的分数可能可以使用电解技术来增加，其中液体玻璃在与两个相反电极的金属电极（“电化学电池”）接触时用作电解质。 该电化学电池由微叉指电极阵列（IDA）组成，该微叉指电极阵列通过将钨溅射到未掺杂的Si或蓝宝石晶片上而产生。 该装置正在通过一系列原型IDA组件开发，该组件将包括IDA本身和高熔化温度陶瓷保持器，该保持器也将IDA保持在所需电连接的位置，并将允许从下面放置金属带，该金属带将用作加热器以使IDA达到工作温度（1,200 ℃至1,300 ℃）。 各种原型配置被设计为优化IDA内的加热和电连续性，并使液体玻璃内产生的离子电流最大化。一个主要的目标是将铅的电离效率提高至少五倍，以提高皮克尺寸铅样品铀铅年龄测定的精度。 第二个目标是开发和商业化一种新的离子源，用于私营部门的热电离质谱仪，热电离质谱仪仍然是高精度同位素比值测定的黄金标准，是目前努力提高U-Pb年龄测定精度的关键。 同位素比测定的精确度由计数的离子数控制，TIMS的主要限制是目前通过从连续加热的金属带表面发射产生热化离子的方法具有低电离效率（~ 1%，很少~10%）。 该提案涉及开发一种新的离子源，该离子源建立在“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，大多数金属原子掺杂到熔融玻璃离子发射体中进行同位素分析，并在蒸发过程中作为中性原子而不是离子释放出来，因此永远不会被输送到仪器的分析仪部分。 该项目的目标是通过将熔体作为电化学电池中的电解质来处理，以增加这些熔体中金属离子的比例。 在所提出的工作中的主要努力将是设计，制造和测试的电化学电池，其中微叉指电极阵列（微IDA）作为工作和反电极，并作为基板上的金属掺杂的硅酸盐沉积，熔融和操纵电诱导电离的金属掺杂剂。 在该方法中，通过将两个电极的相对电势调节到从中性金属去除电子所需的值来诱导金属掺杂剂的电离。 然后在熔融硅酸盐的蒸发过程中将金属离子释放到质谱仪的源。 小的微型IDA电极阵列对于该目的是理想的，因为它们可以完全放置在质谱仪的焦平面中，它们的亚毫米尺寸的电极和电极间隔应该保持被电解质润湿，即使当电解质蒸发并将金属离子释放到质谱仪中时，并且因为微型IDA容易用常规的半导体制造技术根据用户规格制造。 该项目将涉及建立微型IDA设计、衬底和保持器/加热器的组合，这些组合可有效熔化微型IDA表面上的硅酸盐，并使铅电离效率提高2-10倍，这是通过测量离子束强度和持续时间来确定的。目前在科罗拉多大学博尔德分校运行的TIMS。 后一种仪器已经配备了为微型IDA供电所需的定制恒电位仪。

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.