| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | July 18, 2017 |
| Latest Amendment Date: | April 10, 2019 |
| Award Number: | 1664313 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
David Lambert
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | August 1, 2017 |
| End Date: | January 31, 2022 (Estimated) |
| Total Intended Award Amount: | $136,274.00 |
| Total Awarded Amount to Date: | $163,209.00 |
| Funds Obligated to Date: |
FY 2018 = $71,341.00 FY 2019 = $26,935.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
3100 MARINE ST Boulder CO US 80309-0001 (303)492-6221 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
3100 Marine Street Boulder CO US 80303-1058 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Instrumentation & Facilities |
| Primary Program Source: |
01001819DB NSF RESEARCH & RELATED ACTIVIT 01001920DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
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.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Thermal ionization mass spectrometry (TIMS) remains the gold standard for high precision elemental isotopic ratio measurements used throughout the geosciences in geochronology and “tracer” studies. This mass spectrometry technique depends on the measurement of ions that are emitted from a solid sample when heated on a metal surface under high vacuum conditions in the instrument (Fig.1). The fact that ions are created through surface ionization was first recognized over one-hundred years ago and is a process that can be exploited to produce excellent but inefficient sources of thermalized ions. Typically, only one out of one hundred atoms loaded onto the metal ribbon are ionized in this fashion. Our study was designed to develop techniques to improve the ionization efficiencies of thermalized ion sources. We based our approach on work conducted over past twenty years on the mechanism of ion formation in liquid glass ion emitters, a variant of the typical metal ionizers used in TIMS which involves loading a metal sample (eg. Pb) doped in a boro- or phospho-silicate slurry onto a Re-metal ribbon which is then heated to produce emitted metal ions. Previous work suggested that such liquid glass ion emitters produce ions via mechanisms other than, or at least in addition to, surface ionization, including oxidation-reduction reactions within the glass itself. Our project was designed to determine if it is possible to provoke additional formation of Pb+ ions in a molten glass under high vacuum condition using electrochemical techniques. We used a “lab on a chip” approach for this purpose (Fig. 2). We designed and fabricated using photo-lithography techniques a set of micro-interdigitated electrode arrays (IDAs) comprised of either tungsten or titanium sputtered onto a silicon or sapphire wafer. After dicing the wafers, the IDAs were placed into a custom designed and fabricated ceramic holder that allowed the wafer to be conductively heated below by a Re-ribbon heater and to be connected electrically to an external potentiostat. After several design iterations, we were successful in melting Pb-doped borosilicate glass loaded onto an IDA surface constructed on a sapphire wafer and were able to generate from this glass standard cyclic voltammograms in a high vacuum chamber that simulated the source of a standard TIMS. To our knowledge, this is the first time that electrochemical experiments using molten glass as an electrolyte have been successfully conducted on an IDA at high temperatures (>1,000 oC) and under high vacuum conditions. Our voltammograms show a clear anodic current peak that we interpret as the oxidation of neutral Pb atoms to Pb+(Fig. 3). We then placed this IDA assembly in a TIMS instrument equipped with a custom built potentiostat floated at 10kV that allows the electrochemical experiments to be conducted on the IDA in situ in the mass spectrometer. In this fashion, we demonstrated that increases in Pb+ ion emission are detected in our TIMS at IDA voltages corresponding to the peak anodic currents observed in our voltammograms. This observation confirms our attribution of the anodic current peak to the formation of Pb+ in the molten glass. However, the ionization efficiency gained by operating the liquid glass emitter as an electrochemical cell and at electrode potentials that provoke peak Pb+ formation was modest (<10%). Given this observation, we then investigated the effect of changing the relative amounts of metal and non-conducting sapphire that underly a Pb doped liquid glass emitter, when connected to a normal current supply. These experiments suggest that proportion of metal surface area underlying the molten glass ion emitter is the main factor influencing Pb+ emission, and not oxidation-reduction reactions involving Pb that occur in the glass, itself (Fig. 4). We conclude that although we can improve Pb ion emission from liquid glass by inducing electrochemical reactions using a micro-IDA, liquid glass emitters are ultimately surface ionizers and the main controls on ion production from the glasses are processes occurring on the liquid glass/metal interface. Efforts to improve ionization of efficiencies from liquid glass ion emitters should therefore concentrate on manipulating those factors that control surface ionization efficiency, such as the metal surface work function and the overall surface area in contact with the glass emitter.
Last Modified: 06/01/2022
Modified by: G. Lang Farmer
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