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 134Cs/137Cs has been widely used as a tracer for contamination source identification. However, because of the short half-life of 134Cs (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 135Cs (t 2/1 = 2 × 106 y) and the atomic ratio of 135Cs/137Cs, 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 (104–105) 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 (135Ba+ and 137Ba+) and polyatomic interferences (95 Mo40Ar+, 97 Mo40Ar+, 119Sn16O+, and 121Sb16O+) produced by sample matrix ions. The high abundance sensitivity (10–9 for the 135Cs/133Cs ratio) provided by ICP-MS/MS allowed reliable analysis of 135Cs and 137Cs 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 135Cs and 137Cs isotopes in environmental samples (soil, litter, and lichen) collected after the FDNPP accident for contamination source identification.