Exploration of Metal-Organic Frameworks as Ion Emitters for Uranium Analyses by Thermal Ionization Mass Spectrometry

For many decades, thermal ionization mass spectrometry (TIMS) has been heavily used for high sensitivity and high precision actinide isotope measurements in nuclear and geological materials. However, a number of these methods severely suffer from poor sample utilization efficiencies (SUEs) of << 1%. Such efficiencies hinder the ability to measure ultra-trace actinide isotopes by TIMS but can be improved by employing an ion source method, such as the resin bead and the porous ion emitter. While these ion source methods can achieve >1% SUEs, the methods have shown to be challenging to replicate, depend upon the operator, and/or require customized equipment. Recently, our group introduced the novel use of metal-organic frameworks (MOFs) as nano-porous ion emitters (nano-PIEs) for neodymium isotope analyses. The application of MOFs is advantageous as MOFs can easily be engineered into desired physical and chemical structures (i.e., surface area, porosity, order of structure, metal incorporation, organic and inorganic linkers, conductivity, and sorption) to promote higher ionization efficiencies.

Here, we exploit MOFs' tunability to explore a series of frameworks, compositions, and linkers to improve and optimize the ionization efficiency of uranium at low pico-grams (= 10-12 g). Our results on uranium emission using MOF-74, MOF-253, and MIL-101 structures indicate that 1) these frameworks are better ion producers for uranium than the direct loading onto a filament, 2) uranium emission is a function of the amount of MOF loaded on a filament and depends on filament ramp conditions, and 3) MOFs provide stable redox conditions as linkers and carbon additives (e.g., benzene) have negligible effect on formation of uranium molecular species and, therefore, little effect on SUEs. To date, we have achieved 0.46% efficiency with Zn-MOF-74, a MOF synthesized from zinc and organic ligand 2,5-dihydroxybenzene-1,4-dicarboxylate to form channels of a one-dimensional honeycomb structure with high surface area.