SI-traceable quantification of 135Cs in 137Cs solution for 135Cs standardization†

Abstract

The quantification of long-lived radiocesium, ¹³⁵Cs, is essential for assessing the long-term safety of high-level radioactive waste repositories. However, the lack of certified reference materials for ¹³⁵Cs measurement via mass spectrometry presents a significant challenge as the demand for universally reliable analytical results grows. This study investigated a ¹³⁷Cs solution generated from uranium fission as a candidate for a Cs isotope standard reference material, assuming that it contains a comparable concentration of ¹³⁵Cs relative to ¹³⁷Cs. The mass fractions of Cs isotopes (¹³³Cs, ¹³⁵Cs, and ¹³⁷Cs) in the ¹³⁷Cs solution were determined with SI traceability using triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS), along with the Cs molar mass and isotopic composition. Precise measurements of ¹³⁵Cs (20 pg g⁻¹) were achieved with a standard uncertainty of 0.1–0.3% and a sensitivity of approximately 150 cps per pg g⁻¹, using the N₂O gas mode to eliminate Ba interference effectively. The mass discrimination in the ¹³⁵Cs/¹³³Cs molar ratio obtained via ICP-MS/MS was corrected using the ¹³⁷Cs/¹³³Cs molar ratio, which was traceably determined to the SI unit, as a reference value. The resulting mass fraction and isotopic composition of ¹³⁵Cs were determined to be 21.43 ± 0.75 ng g⁻¹ (k = 2) and 0.1891 ± 0.0057 (k = 2), respectively. Based on these ¹³⁵Cs/¹³³Cs and ¹³⁷Cs/¹³³Cs molar ratios, the ¹³⁵Cs/¹³⁷Cs molar ratio was established at 1.008 ± 0.048 (k = 2). This study presents the first verified SI-traceable isotopic composition for Cs.