Procedures for precise measurements of 135Cs/137Cs atom ratios in environmental samples at extreme dynamic ranges and ultra-trace levels by thermal ionization mass spectrometry

Publication date: 1 November 2017
Source:Talanta, Volume 174
Author(s): James A. Dunne, David A. Richards, Hsin-Wei Chen
Determination of ¹³⁵Cs/¹³⁷Cs atom ratios has the potential to be a powerful tool for nuclear forensics and monitoring environmental processes. We present optimized chemical separation techniques and thermal ionization mass spectrometry (TIMS) protocols to obtain precise ¹³⁵Cs/¹³⁷Cs atom ratios for a range of environmental sample types. We use a combination of double AMP-PAN separation and Sr-spec resin column purification to yield excellent separation from the alkali metals (Rb separation factor > 600), which normally suppress ionization of Cs. A range of emission activators for the ionization of Cs were evaluated and glucose solution yielded the optimal combination of a stable Cs⁺ beam, minimal low-temperature polyatomic interferences and improved ionization efficiency. Mass-spectrometric determination of low abundance ¹³⁵Cs and ¹³⁷Cs is compromised by the presence of a very large ¹³³Cs⁺ beam, which may be scattered and cause significant spectral interferences. These are explored using multi-static Faraday cup – ion counter methods and a range of energy filter settings. The method is evaluated using environmental samples and standards from regions affected by fallout from Chernobyl (IAEA-330) and Fukushima nuclear disasters. Where the intensity of ¹³³Cs⁺ is large relative to ¹³⁵Cs⁺ and ¹³⁷Cs⁺ (< 30cps), minor polyatomic interferences need to be considered. In the absence of a standard with ^{135, 137}Cs/¹³³Cs < 1 × 10⁻⁸, we explored the reproducibility of ¹³⁵Cs/¹³⁷Cs atom ratios at these high dynamic ranges and extremely low abundance (¹³⁷Cs ≈ 12 fg g⁻¹) for sediments from an estuarine setting in SW England, UK.

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