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

摘要

Determination of Cs-135/Cs-137 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 Cs-135/Cs-137 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 Cs-135 and Cs-137 is compromised by the presence of a very large Cs-133(+) 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 Cs-133(+) is large relative to Cs-135(+) and Cs-137(+) (< 30 cps), minor polyatomic interferences need to be considered. In the absence of a standard with (CS)-C-135, 137/(CS)-C-133 < 1 x 10(-8), we explored the reproducibility of Cs-135/Cs-137 atom ratios at these high dynamic ranges and extremely low abundance (Cs-137 approximate to 12 fg g(-1)) for sediments from an estuarine setting in SW England, UK.