Anion-Exchange Fibers for Improved Sample Loading in Ultra-Trace Analysis of Plutonium by Thermal Ionization Mass Spectrometry

Evaluation of Graphene Oxide as a Thermal Ionization Enhancer for Plutonium in TIMS Measurement

Thermal ionization mass spectrometry (TIMS) has been extensively employed for the assessment of plutonium (Pu) isotopes in nuclear forensics and environmental monitoring. Recently, great efforts have been made to improve the ionization efficiency (IE) of Pu to achieve better accuracy and precision for trace-level analysis. Herein, the thermal ionization enhancement effect for plutonium of graphene oxide (GO) was investigated and the corresponding mechanism was discussed. The GO layers were homogeneously mounted on the filament's central surface to promote pg-level Pu ion emission. With the excellent structural property of GO, a greatly promoted ionization efficiency of 0.44% for Pu was obtained, and the initial ionization temperature for Pu was remarkably reduced from 1610 to 1390 °C. Average boosts in IE compared to the classical double-filament mode and graphite-loaded single-filament mode were 1640 and 520%, respectively. The analytical accuracy and precision based on the GO-loaded single-filament mode were validated using Pu isotopic certified reference materials. This work demonstrates the excellent property of GO as an ion source additive for Pu ionization, as it provided an interface for the promotion of energy transfer and Pu carbide formation. The operation of GO loading is quite simple and can be finished within 5 min. This rapid filament carburization approach has great potential for improving the measurement precision of trace-level plutonium isotopes and can be applied in nuclear safeguards, nuclear forensics, and environmental monitoring.

Task-Specific Supported Liquid Membrane for Actinide Assay in Aqueous Streams by Thermal Ionization Mass Spectrometry

Thermal ionization mass spectrometry is the most commonly used technique for the determination of Pu isotopic composition and concentration in complex matrices but involves multiple steps including sample pretreatment, removal of matrix, preconcentration of Pu, and loading on a rhenium filament for TIMS analysis. The present work reports the synthesis of the N,N'-dioctyl-α-hydroxyacetamide (DOHA) functionalized supported liquid membrane that offered dual functions: (i) matrix elimination and/or preconcentration of actinides from complex aqueous samples and (ii) served as a loading substrate for TIMS analysis. The ligand composition in the membrane can be tuned aiming either for selective preconcentration of Pu from an aqueous matrix or for bulk removal of actinides. The membrane, impregnated with 0.2 M DOHA in dodecane, showed very high selectivity for Pu^IV in acidic medium, in the presence of other competing actinides, viz., Am^III, U^VI, and Np^V. The membrane based loading in TIMS improved the sample utilization efficiency and ionization efficiency compared to the conventional solution loading technique, offering Pu analysis from a single Re filament, that served as both vaporization and ionization filament and direct determination of ²³⁸Pu in the presence of ²³⁸U, eliminating the requirement of alpha spectrometry. It was possible to achieve >80% reduction in analysis time and >95% reduction in secondary waste generation by the SLM-TIMS method, compared to conventional TIMS involving Pu purification by anion exchange resin. Pu concentrations in seawater and groundwater samples, synthetic urine, and dissolver solution of irradiated fuel were determined by SLM-TIMS, employing the isotope dilution (ID) technique, with very good accuracy and precision. The membrane, impregnated with 2 M DOHA in dodecane, showed strong affinity for actinides and was successfully employed for the removal of bulk actinides from aqueous samples with more than 96% recovery.

Anion-exchange polymer filament coating for ultra-trace isotopic analysis of plutonium by thermal ionization mass spectrometry

A new sample loading procedure was developed for isotope measurements of ultra-trace amounts of Pu with thermal ionization mass spectrometry (TIMS) that is based on a polymer thin film architecture. The goals were to simplify single filament TIMS sample preparation for Pu, while preserving the sensitivity and accuracy of the resin bead loading method, and to eliminate sample losses experienced with the bead loading method. Rhenium filaments were degassed, dip-coated with a thin (~ 120 nm) hydrophobic base layer of poly(vinylbenzyl chloride) (PVBC), and spotted with an aqueous solution comprising triethylamine-quaternized PVBC and diazabicyclo[2.2.2]octane crosslinker. This procedure formed a toroidal, hydrophilic anion-exchange polymer spot surrounded by the hydrophobic base polymer. The thin film-coated filaments were direct loaded with 10 pg of New Brunswick Laboratory certified reference material (NBL CRM) 128 from a 9 M HCl matrix. Aqueous sample droplets adhered to the anion-exchange polymer spot, facilitating sample loading. Toroidal spots with a thickness of 20-30 µm generated the highest sample utilization, surpassing the sample utilization of the standard bead loading method by 175%. Measured isotopic ratios were in good agreement with the certified value of the ²³⁹Pu/²⁴²Pu ratio for NBL CRM 128. The use of dimpled filaments further aided sample loading by providing a well-shaped substrate to deposit the sample droplet. No sample losses were experienced with the thin film loading method over 65 sample analyses. Finally, polymer coatings suppressed filament aging under atmospheric conditions, enabling the bulk production of filaments with adequate shelf life for future analyses.