Rapid 235U/238U determination by matrix assisted ionization–time-of-flight mass spectrometry†‡

Matrix-assisted ionization (MAI) of inorganic analytes is a nascent research domain that holds promise for rapid, potentially facility-deployable analytical applications. We present results of MAI uranium isotopic analysis (²³⁵U/²³⁸U) obtained on the timescale of minutes utilizing simple sample preparation and an ambient ionization time-of-flight mass spectrometer (ToF MS). Experimental MAI-ToF MS characterization of uranium Certified Reference Materials (CRMs) was used to establish method calibration and validate quantitative ²³⁵U/²³⁸U determination spanning depleted, natural, and low-enriched uranium isotopic compositions. Secondary standard analyses with total uranium mass loadings of 5–500 ng per analysis yield accurate calibrated ²³⁵U/²³⁸U results and relative uncertainties of 4.7–17.2% (approx. ±95% confidence level), with weighted-mean uncertainties approaching 1.5%. This method permits accurate determination of uranium isotopic composition in a sample with uranium content as low as 200 pg for equal atom ²³⁵U:²³⁸U. Instrument detection limits constrain the minimum uranium mass required to identify the presence of highly enriched uranium (HEU ≥20% ²³⁵U) as only 500 pg using the method presented here. MAI-ToF MS quantitation of relatively extreme isotope ratios (²³⁵U/²³⁸U ≤ 0.01) is limited by detection of minor ²³⁵U (LoD 100 pg ²³⁵U/analysis ≈ 10 ng total U/analysis), and subsequent method optimization is anticipated to further reduce these limits. These findings underscore the potential of MAI-ToF MS for isotopic characterization of uranium and other inorganic species for both basic and applied science.