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

摘要

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+ (< 30 cps), minor polyatomic interferences need to be considered. In the absence of a standard with 135, 137Cs/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.