Determination of low-level ¹³⁵Cs and ¹³⁵Cs/¹³⁷Cs atomic ratios in large volume of seawater by chemical separation coupled with triple-quadrupole inductively coupled plasma mass spectrometry measurement for its oceanographic applications

Radioisotopes of cesium are powerful tracer for oceanographic studies. In this work, a novel method was developed for determination of ultra-low level ¹³⁵Cs and ¹³⁷Cs in seawater using triple-quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was pre-concentrated from up to 45 L seawater samples using ammonium molybdophosphate (AMP) adsorption, following a selective leaching of cesium using Sr(OH)₂. The cesium was further purified from interfering elements using AMP-PAN and cation-exchange chromatography. Sr(OH)₂ leaching was found to be an effective approach for selective exchange of cesium from the AMP sorbent without dissolution, which avoids the problem of separation of huge amount of NH₄⁺ and MoO₄²⁻ in the following steps. The decontamination factors for barium and rubidium with the developed method were more than 4 × 10⁷ and 800, respectively. The separated ¹³⁵Cs and ¹³⁷Cs were measured using ICP-MS/MS by employing N₂O as reaction gas to further elimination of isobaric (i.e. ¹³⁵Ba and ¹³⁷Ba) and polyatomic ions interferences. A detection limit of 1.5 × 10⁻¹⁶ g L⁻¹ for ¹³⁵Cs in seawater was achieved. The concentrations of ¹³⁵Cs in seawater from Baltic Sea, Danish straits and Roskilde Fjord were determined using the developed method to identify the sources of ¹³⁵Cs, the water masses exchange in this region was investigated using ¹³⁵Cs and ¹³⁷Cs.