Quantitative analysis of radium-226 at the microscale by NanoSIMS

Abstract

This study presents an analytical protocol for the quantitative imaging of ²²⁶Ra by Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), based on the synthesis and characterization of barite reference materials enriched in ²²⁶Ra (2.89 and 9.65 µg g⁻¹), ²³²Th (2120 µg g⁻¹), and Pb (85, 755 and 8080 µg g⁻¹). This methodological development addresses key analytical challenges related to the low abundance of ²²⁶Ra and spectral interferences. We identified ⁸⁸Sr¹³⁸Ba⁺ and ²⁰⁸Pb¹⁸O⁺ as potential polyatomic interferences for ²²⁶Ra⁺. However, we demonstrated that these species do not significantly impact the ²²⁶Ra⁺ signal when appropriate mass resolution and precise peak alignment are applied. Thanks to the use of homemade ²²⁶Ra–barite reference materials, we demonstrated that NanoSIMS measurements can achieve reproducibility ranging from 6% to 12% (2σ) across analytical sessions, depending on the Ra content and the size of the analyzed area. Comparison with known concentrations confirmed the accuracy of the ²²⁶Ra⁺/¹³⁸Ba⁺ ratio as a quantitative proxy for radium barite content. A conservative quantification limit achieved for ²²⁶Ra is 0.135 µg g⁻¹, making this approach applicable to samples from the natural environment. These results demonstrate that NanoSIMS provides accurate and reproducible quantitative imaging of ²²⁶Ra at the micrometric scale using matrix-matched internal reference materials. This method opens up new opportunities for studying ²²⁶Ra distribution in samples from the natural environment at the mineral scale.