Determination of 135Cs and 135Cs/137Cs Atomic Ratio in Environmental Samples by Combining Ammonium Molybdophosphate (AMP)-Selective Cs Adsorption and Ion-Exchange Chromatographic Separation to Triple-Quadrupole Inductively Coupled Plasma-Mass Spectrometry

Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011, the activity ratio of Cs-134/Cs-137 has been widely used as a tracer for contamination source identification. However, because of the short half-life of Cs-134 (2.06 y), this tracer will become unavailable in the near future. This article presents an analytical method for the determination of the long-lived Cs-135 (t(2/1) = 2 x 10(6) y) and the atomic ratio of Cs-135/Cs-137, as a promising geochemical tracer, in environmental samples. The analytical method involves ammonium molybdophosphate (AMP)-selective adsorption of Cs and subsequent two-stage ion-exchange chromatographic separation, followed by detection of isolated radiocesium isotopes via triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS). The AMP-selective adsorption of Cs and the chromatographic separation system showed high decontamination factors (10(4)-10(5)) for interfering elements, such as Ba, Mo, Sb, and Sn. Using ICP-MS/MS, only selected ions enter the collision/reaction cell to react with N2O, reducing the isobaric interferences (Ba-135(+) and Ba-137(+)) and polyatomic interferences ((MoAr+)-Mo-95-Ar-40, (MoAr)-Mo-97-Ar-40, (SnO+)-Sn-119-O-16, and (SbO+)-Sb-121-O-16) produced by sample matrix ions. The high abundance sensitivity (10(-9) for the Cs-135/Cs-133 ratio) provided by ICP-MS/MS allowed reliable analysis of Cs-135 and Cs-137 isotopes with the lowest detection limits ever reported by mass counting methods (0.01 pg mL(-1) and 0.006 pg mL(-1), respectively). The developed analytical method was successfully applied to the determination of Cs-135 and Cs-137 isotopes in environmental samples (soil, litter, and lichen) collected after the FDNPP accident for contamination source identification.