Strontium is determined from milk powder. Milk powder is a good starting material because to it has been concentrated from large amount of milk. Organic matter is incinerated and ash dissolved into small amount nitric acid. Strontium is separated from alkali metal with phosphate precipitation. Removal of potassium is of special interest since it interferes with strontium separation with extraction chromatography by using SrResin (Eichrom/Triskem). From the eluate of SrResin strontium is precipitated as strontium carbonate for yield determination, dissolved in acid and measured with liquid scintillation counting.
90Sr (T1/2: 28.8 a, Emax: 0.54 MeV) decays with β-emission to the daughter nuclide 90Y (T1/2: 64.0 h, Emax: 2.28 MeV) which starts to grow in to a sample immediately after the separation of strontium. The β-spectra of 90Sr and 90Y are overlapping and therefore the ingrown 90Y interferes with the measurement of 90Sr activity. The 90Sr-source has to be measured either immediately after the separation of strontium or when yttrium and strontium are in equilibrium in the sample in about two weeks. In equilibrium the activities of 90Sr and 90Y are the same. Complete achievement of equilibrium is not necessary since the ingrowth percentage can be calculated from the time difference between the separation and the measurement. In this exercise strontium is measured after the ingrown of 90Y with a low background liquid scintillation counter (Quantulus, Wallac, Perkin Elmer). The counter is calibrated with a 90Sr standard solution in which 90Y is in equilibrium with 90Sr.