Ultra-trace determination of the 135Cs/137Cs isotopic ratio by thermal ionization mass spectrometry with application to Fukushima marine sediment samples

Abstract

The isotopic ratio of ¹³⁵Cs/¹³⁷Cs has been highlighted as a useful long-term tracer for radioactive contamination source identification since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Determination of this ratio in environmental samples is still challenging due to the extremely low levels (<1 pg g⁻¹) of ¹³⁵Cs and ¹³⁷Cs. When applying TIMS for analysis of the ¹³⁵Cs/¹³⁷Cs ratio, major problems arise from ion ionization depression effects caused by the sample matrix elements and the ionization inhibitor (Rb). Moreover, isobaric and polyatomic species generate interfering signals at the masses of interest. Therefore, a chemical separation procedure with high separation factors for the matrix and interfering elements must be established. In this work, we report a novel method for determination of the ¹³⁵Cs/¹³⁷Cs isotopic ratio in environmental samples. A combination of single AMP-PAN column separation and ion exchange chromatography purification was used for sample preparation. High separation factors (10⁴ to 10⁸) for the major matrix and interfering elements (Ca, Fe, Mg, Na, Ba, etc.) were achieved. Due to the separation of Cs and Rb (separation factor > 500), the ion ionization suppression effect of Cs was negligible. This method is applicable for the measurement of isotopic ratios of radiocesium at femtogram levels. The precision of the measurements of the ¹³⁵Cs/¹³⁷Cs ratios was mostly better than 10% for samples containing as little as 10 fg ¹³⁷Cs. Reference materials were measured and the results were compared with literature values. This method was successfully applied for the measurement of ¹³⁵Cs/¹³⁷Cs isotopic ratios in marine sediments collected in the western North Pacific after the FDNPP accident for radioactivity assessment in a marine environment.