PRECISE DETERMINATION OF 89Sr AND 90Sr/90Y IN VARIOUS MATRICES: THE LSC 3-WINDOW APPROACH

ABSTRACT. A fast method is presented for determining radioactive isotopes of Sr in environmental and bioassay samples using extraction chromatography and liquid scintillation counting performed in a 3-window region under normal counting mode, which yields the best sensitivity values (E2/B). Prior to application of radiochemical separation procedures, all samples are spiked initially with almost carrier-free 85S tracer for calculation of the chemical recovery. After i) preconcentration of Sr via oxalic acid (soil, grass) or carbonate precipitation (urine, waste water) and ii) redissolution in nitric acid and purification using SrSpec resin, iii) LS counting is performed in a 3-window region, i.e. ranging from 0–10 keV (85Sr spike); 10–300 keV, the maximum energy of 90Sr on the LS scale (for 90Sr + 89Sr); and 300–1200 keV (for 89Sr + ingrowing 90Y). Constant quench conditions are obtained by preparing cocktail solutions consisting of a mixture of 2 mL toluene sulfonic acid (in which the Sr fraction is dissolved) and 18 mL Ultima Gold LLT scintillation liquid. This mixture results in highly reproducible β spectra, i.e. the counting efficiencies of the different isotopes remain constant and independent on the starting matrix taken for anal-ysis. The respective counting windows were selected to obtain optimum sensitivity for the 2 radioactive Sr isotopes, i.e. to consider situations in which the activity of 90Sr significantly exceeds that of 89Sr and vice versa. Using low-level LS spec-trometers such as the PerkinElmer Tri-Carb 2550TR/AB, detection limits for both Sr radioisotopes of <0.1 Bq/L are obtained in a 5 × 100-min counting interval if 0.2-L water aliquots are processed. The 3-isotope/3-window approach was validated internally on various matrices and by participation in an intercomparison exercise on artificial reactor water samples (Germa