Development of a Simplified Cavity Thermal Ionization Source for Geoscience Applications

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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.