Strontium-90 determination in milk ash by Cherenkov and gas-flow proportional counting: A comparison of three radiochemical separations

Strontium-90 (⁹⁰Sr) is a long-lived fission product whose determination in milk is analytically demanding. It is a pure beta emitter and must be separated from a large excess of stable calcium. Three radiochemical separations were compared for ⁹⁰Sr determination in a milk-ash matrix: solvent extraction with tributyl phosphate (TBP), extraction chromatography on Sr Resin, and molecular-recognition separation on AnaLig® Sr-01. Eight raw-milk-aliquots equivalent to raw milk were spiked at a nominal ⁹⁰Sr activity concentration of 10 Bq/L and analysed by all three methods in a paired design. The Sr Resin and AnaLig® Sr-01 eluates were measured by Cherenkov counting of the ingrowing daughter ⁹⁰Y on a low-level liquid scintillation analyser, whereas the TBP procedure used gas-flow proportional counting of the separated ⁹⁰Y. The Cherenkov counting efficiency was characterised against the spectral index of the sample across a graded iron(III) colour-quench series, which guided the choice of working energy window. Mean chemical recoveries were approximately 90% for AnaLig® Sr-01, 85% for Sr Resin and 75% for TBP. The three methods yielded statistically indistinguishable activity concentrations, as confirmed by repeated-measures analysis of variance, the Friedman test and pairwise comparisons, with method minimum detectable activity concentrations of 0.296 – 0.332 Bq/L, far below the regulatory levels for dairy produce. The principal differences were therefore operational rather than analytical: TBP suits aged, equilibrated samples; Sr Resin is a validated rapid option for emergencies; and AnaLig® Sr-01 is attractive for routine use through sorbent regeneration and reduced waste generation.