Determination of Ultralow Level 135Cs and 135Cs/ 137Cs Ratio in Environmental Samples by Chemical Separation and Triple Quadrupole ICP-MS

An analytical method was developed for the determination of ultralow level Cs-135 in environmental samples by chromatographic separation of cesium with AMP-PAN and AG50W-X8 columns and sensitive measurement of cesium isotopes with triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was simply released by acid leaching using aqua regia from environmental solid samples and preconcentrated on AMP-PAN column. The cesium adsorbed on the column was effectively eluted with NH4Cl solution without dissolving the AMP. The excessive amount of NH4Cl in the eluate was removed by sublimation in the presence of small amount of LiCl. The remaining barium and other interfering elements such as Mo, Sn, Sb, and Li were efficiently removed using cation exchange chromatography (AG50W-X8). The decontamination factors of this procedure are above 4 X 10(7) for barium and 4 X 10(5) for molybdenum; the chemical yields of cesium are more than 85% for samples of less than 10 g. This method enables to separate cesium from large size of samples for the determination of ultralow level Cs-135, avoiding the problem of removal of a huge amount of Mo in the dissolved AMP. Intrinsic Cs-137 in the environmental samples measured by gamma spectrometry before and after separation was used as internal isotope dilution standard for quantitative determination of Cs-135 without complete release and recover of radiocesium. The interference of barium ( Ba-135 and Ba-137) to the ICP-MS measurement of Cs-135 and Cs-137 was further suppressed to 8 X 10(-5 )by using N2O as the reaction gas in ICP-MS/MS at a flow rate of 0.7 mL/min, so a total suppression of 2 X 10(-12) for Ba was achieved, making the isobaric interference of Ba isotopes to the measurement of Cs-135 and Cs-137 in environmental samples negligible. A detection limit of 9.1 X 10(-17) g/g for Cs-135 and Cs-137 was achieved for 60 g samples. The developed method was validated by analysis of standard reference materials (IAEA-375, IAEA-330, and IAEA-385) and successfully applied for the determination of Cs-135 concentrations and Cs-135 and Cs-137 ratios in soil samples collected from Denmark, Sweden, and Ukraine. The Cs-135 and Cs-137 isotopic ratios in Danish soil (2.08-2.68) were significantly higher than that from Sweden and Ukraine (0.65-0.71), indicating different sources of radiocesium. This work demonstrated the application of Cs-135 and Cs-137 as a unique fingerprint for discriminating the sources of radioactive contamination and estimating their contribution to the total inventory, which will be useful for nuclear forensics and environmental tracer studies.