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More than Five Percent Ionization Efficiency by Cavity Source Thermal Ionization Mass Spectrometry for Uranium Subnanogram Amounts

Trinquier, Anne, Maden, Colin, Fauré, Anne-Laure, Hubert, Amélie, Pointurier, Fabien, Bourdon, Bernard, Schönbächler, Maria
Analytical chemistry 2019 v.91 no.9 pp. 6190-6199
environmental surveys, forensic sciences, ion exchange, ionization, ions, isotopes, mass spectrometry, prototypes, radionuclides, reference standards, uranium
Numerous applications require the precise analysis of U isotope relative enrichment in sample amounts in the subnanogram to picogram range; among those are nuclear forensics, nuclear safeguards, environmental survey, and geosciences. However, conventional thermal ionization mass spectrometry (TIMS) yields U combined ionization and transmission efficiencies (i.e., ratio of ions detected to sample atoms loaded) of less than 0.1% or 2% depending on the loading protocol, motivating the development of sources capable of enhancing ionization. The new prototype cavity source TIMS at ETH Zürich offers improvements from 4 to 15 times in combined ionization and transmission efficiency compared to conventional TIMS, yielding up to 5.6% combined efficiency. Uranium isotope ratios have been determined on reference standards in the 100 pg range bound to ion-exchange or extraction resin beads. For natural U standards, n(²³⁵U)/n(²³⁸U) ratios are measured to relative external precisions of 0.5–1.0% (2RSD, 2 < n < 11, conventional source) or 2.0% (2RSD, n = 6, cavity source) and accuracies of 0.2–0.7% (conventional source) or 0.4–0.9% (cavity source). Meanwhile, n(²³⁴U)/n(²³⁸U) ratios are determined to relative external precisions of 1.7–3.6% (2RSD, 2 < n < 11, conventional source) or 5.6% (2RSD, n = 6, cavity source) and accuracies of 0.1–2.5% (conventional source) or 0.5–8.3% (cavity source), which would benefit further from in-run organic interference and peak tailing corrections.