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 standards 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 and demonstrated that they do not significantly affect the 226Ra+ signal when mass resolution and peak alignment are carefully optimized. We demonstrated that 226Ra is co-precipitated with Ba in sulfate standard matrices and that NanoSIMS measurements have reproducibility of 6% and 7% (2σ) across analytical sessions depending on Ra content. 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 environmental samples. These results demonstrate that NanoSIMS provides accurate and reproducible quantitative imaging of 226Ra at the micrometric scale using matrix-matched internal standards. This method opens new opportunities for studying 226Ra distribution in environmental samples at the mineral scale.