Long-term Experiments for the Determination of the Influence of Storage Media and Storage Conditions on the Alteration of Uranium Oxide Microparticle Reference Materials to Identify Optimal Storage Conditions

The IAEA (International Atomic Energy Agency) and its worldwide Network of qualified Analytical Laboratories (NWAL) are using uranium oxide-based microparticulate reference materials to strengthen the IAEA’s quality control system for particle analyses via mass spectrometric analysis methods e.g., Large Geometry–Secondary Ion Mass Spectrometry (LG-SIMS) and Thermal Ionization Mass Spectrometry (TIMS). The microparticles are intended for instrument calibration, method development and validation as well as for application in interlaboratory comparison exercises and additionally to support the pressing demand to build-up new NWAL capabilities. To fulfill these very special needs these reference materials must be well-defined in size and shape as well as in isotopic and elemental composition.  A core requirement for the distribution of a potential reference material is to guarantee a practical shelf-life. Previous studies on structure and shape of uranium oxide microparticles showed an alteration process leading to the formation of uranium hydroxide like schoepite. However, the formation of aqueous uranium oxide phases could lead to analytical difficulties, particularly in measurements with LG-SIMS. The very high vacuum applied in the device can lead to the decomposition of the aqueous phases and thus to the destruction of the reference particles. A reliable measurement of these particles in the LG-SIMS is then impossible or only with great effort. To further investigate this alteration process, a systematic shelf-life study of aerosolbased uranium oxide microparticles (produced at the NWAL laboratories at Forschungszentrum Jülich) has been launched in 2021. In a first part of this study, the influence of different atmospheric conditions was evaluated. Humid air, normal laboratory air, Ar-atmosphere in a desiccator and a storage of the microparticles in a lab furnace at 90°C were tested. To this purpose, the differently stored uranium oxide microparticles from the same production batch were deposited on Glass-like Carbon Disks (GCDs) and stored under these different atmospheric conditions. The second part of the shelf-life study is related to the long-term stability in different suspension media. To transfer microparticles after the production into an alcoholic suspension offers a very high degree of flexibility for storage and further use of the microparticles as well as homogeneity with respect to the particle distribution on different substrates. As suspension media, only anhydrous solvents with high degree of purity were used (ethanol, 2-propanol, n-butanol, tert-butanol).  The uranium oxide particles used in both approaches were periodically measured using Scanning Electron Microscopy (SEM) and μ-Raman Spectroscopy for structural as well as LG-SIMS for isotopic investigation. The combination of these aforementioned methods allows to check the long-term stability of uranium microparticles in suspension and under different atmospheric conditions as well as the effect of the possible alteration on the analytics and allows consequently to identify optimal storage conditions for potential reference materials. In this paper recent results of atmospheric and suspension shelf-life studies will be discussed.