Determination of Ultratrace Level 135Cs and 135Cs/137Cs Ratio in Small Volume Seawater by Chemical Separation and Thermal Ionization Mass Spectrometry

The atomic ratio of Cs-135/Cs-137 is a powerful fingerprint for distinguishing the source terms of radioactive contamination and tracing the circulation of water masses in the ocean. However, the determination of the Cs-135/Cs-137 ratio is very difficult due to the ultratrace level of Cs-137 (<0.02 mBq/m(3)) and Cs-137 (<2 Bq/m(3)) in the ordinary seawater samples. In this work, a sensitive method was developed for determination of Cs-135 concentration and Cs-135/Cs-137 ratio in seawater using chemical separation combined with thermal ionization mass spectrometry (TIMS) measurement. Cesium was first preconcentrated from seawater using ammonium molybdophosphate-polyacrylonitrile column chromatography and then purified using cation exchange chromatography to remove the interferences. With this method, decontamination factors of 6.0 x 10(6) for barium and 1800 for rubidium and a chemical yield of more than 60% for cesium were achieved. By using glucose as an activator, the ionization efficiency of cesium was significantly improved to 50.6%, and a constant high current of Cs+ (20 V) can be maintained for more than 180 min, which ensures sensitive and reliable measurement of low level Cs-135 and Cs-137. Detection limits of 4.0 X 10(-17)g/L for both Cs-135 and Cs-137 for 200 mL seawater were achieved, which enables the accurate determination of Cs-135 concentration and Cs-135/Cs-137 ratio in a small volume of seawater samples (<200 mL). The developed method has been validated by analysis of seawater reference material IAEA-443. Seawater samples collected from the Greenland Sea, Baltic Sea, and Danish Straits have been successfully analyzed for Cs-135 concentrations and Cs-135/Cs-137 ratios, and the results showed that Cs-135 concentrations in the seawater of the Baltic Sea is much higher than that in the Greenland Sea, which is attributed to the high deposition of Chernobyl accident derived radiocesium in the Baltic Sea region.