Isotope-Specific Quantitative Mapping of ⁹⁰ Sr at Subfemtogram Levels via Online Isotope Dilution and Laser Ablation Inductively Coupled Plasma Tandem Mass Spectrometry

The quantitative mapping of radioisotopes in solids is crucial for understanding their microscale localization and migration. However, conventional radiometric and mass spectrometric techniques are only capable of bulk measurements without spatial resolution. Imaging plates provide spatial resolution, but they lack selectivity and cannot quantify radioisotopes that only emit beta radiation, such as ⁹⁰Sr. Here, the first system for isotope-specific quantitative mapping of a pure beta emitter is demonstrated, integrating online isotope dilution (ID) with laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS). In a case study, ultratrace levels of ⁹⁰Sr were accurately quantified without requiring matrix-matched certified reference materials (CRMs) for calibration or chemical separation. Severe isobaric/polyatomic interference (e.g., ⁹⁰Zr⁺, ⁷⁴Ge¹⁶O⁺, ⁸⁹Y¹H⁺, and ⁵⁸Ni¹⁶O₂⁺) was efficiently suppressed by O₂ reaction in a dynamic reaction cell, which greatly improved the abundance sensitivity and allowed subfemtogram amounts of ⁹⁰Sr to be quantitatively imaged. The background equivalent concentration (BEC) was 0.14 ng g⁻¹ (equivalent to 7.0 × 10² Bq g⁻¹) per data point, and the measurement time was within 1 s. Validation with CRMs and ⁹⁰Sr samples confirmed accuracy of 88-116% relative to reference values. The proposed system is a versatile platform that can be applied to the practical mapping of radioactive solids, which has broad implications for nuclear waste disposal, environmental remediation, and radiation protection.