Direct isotopic analysis of solid uranium particulates on cotton swipes by microextraction-ICP-MS

Direct isotope ratio analysis of solid uranium particulates on cotton swipes was achieved using a solution-based microextraction technique, coupled to a quadrupole inductively coupled plasma - mass spectrometer (ICP-MS). This microextraction-ICP-MS methodology provides rapid isotopic analysis which could be applicable to nuclear safeguards measurements. Particulates of uranyl nitrate hexahydrate (UO₂(NO₃)₂·6H₂O) and uranyl fluoride (UO₂F₂) ranging from 6 μm to 40 μm in length were transferred to cotton swipes with a particle manipulator. The microextraction probe then delivers a 5% nitric acid (HNO₃) solvent onto the swipe surface to extract the uranium species. The extracted sample is then delivered to the ICP-MS for isotopic determination. The majority of uranium signal (∼99% and ∼94% for UO₂(NO₃)₂·6H₂O and UO₂F₂, respectively) was detected in the first 15 s extraction, while subsequent extractions on the same location had low or no U signal, suggesting near complete removal of the solid uranium compounds from the swipe surface. Ten samples (for each of the uranium compounds), were analyzed for their isotopic composition. For UO₂(NO₃)₂·6H₂O, the determined isotope ratios resulted in a % relative difference (% RD) from the referenced isotope ratios of 0.97, 1.0, and 7.3% for ²³⁴U/²³⁸U, ²³⁵U/²³⁸U, and ²³⁶U/²³⁸U, respectively. The % RD of the UO₂F₂ isotope ratios were 1.9 and 0.60% for ²³⁴U/²³⁸U and ²³⁵U/²³⁸U, respectively. The preliminary limits of detection were determined to be 0.002, 0.4, and 60 pg for ²³⁴U, ²³⁵U and ²³⁸U, respectively This work demonstrates that microextraction ICP-MS is a rapid and sensitive method that could directly determine uranium isotope ratios of UO₂(NO₃)₂·6H₂O and UO₂F₂ particulates on cotton swipes.