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 135 Cs/ 137 Cs 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 135 Cs/ 137 Cs ratio is very difficult due to the ultratrace level of 135 Cs (<0.02 mBq/m 3 ) and 137 Cs (<2 Bq/m 3 ) in the ordinary seawater samples. In this work, a sensitive method was developed for determination of 135 Cs concentration and 135 Cs/ 137 Cs 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 × 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 135 Cs and 137 Cs. Detection limits of 4.0 × 10 –17 g/L for both 135 Cs and 137 Cs for 200 mL seawater were achieved, which enables the accurate determination of 135 Cs concentration and 135 Cs/ 137 Cs 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 135 Cs concentrations and 135 Cs/ 137 Cs ratios, and the results showed that 135 Cs 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.