Ultra-trace ²³⁹Pu and ²⁴⁰Pu determination in soils using optimized leaching, vacuum-assisted TEVA separation, and isotope dilution ICP-MS

The determination of ultra-trace ²³⁹Pu and ²⁴⁰Pu isotopes such as those in the Southern Hemisphere remains a challenge in soils. Most techniques have been developed for relatively higher Pu activity concentrations, which are not suitable for soils with low Pu content. This study aimed to develop an optimized leaching and separation procedure for soils collected from a subtropical catchment in Brazil, and to evaluate its performance against a fusion digestion procedure with ICP-MS. We carried out multiple adjustments to an established procedure such as increasing the acid volume in the TEVA (TEtraValent Actinides and technetium) resin separation steps, using Fe(II) for redox stabilization, reducing sample mass, and increasing the flow rate through a vacuum box. While Pu activities and ratios were generally consistent across methods, our results revealed that using Fe(II) and increasing the acid volume in the separation step with an increased flow rate led to better method performance. The optimal procedure, named the “PULSE method” (Plutonium extraction from Soils at Low concentrations with high prEcision), achieved a high Pu recovery rate of 69 ± 8% and a mean ²³⁸U separation factor of 1.29 × 10⁴. Measurement uncertainties were kept below 10% and the detection limits for ²³⁹Pu and ²⁴⁰Pu amounted to 0.0027 Bq/kg and 0.0008 Bq/kg, respectively. Thus, the PULSE method is a practicable and scalable method that allows for a reliable determination of Pu isotopes, which are considered long-term alternative tracers to shorter-lived radionuclides like ¹³⁷Cs for soil and sediment redistribution processes.