Use of liquid scintillation counting for fast determination of 89Sr and 90Sr in milk.

This paper describes a method of reproducibly recovering strontium (Sr) in 90.4% yield from 2-litre samples of raw milk in an analysis time of 5-8 h. All species which may interfere with the measurement of 89Sr and 90Sr are effectively removed; decontamination factors for Ca, Y, Ba and Cs were found to be adequate, and the role of other possible interferences is discussed. The Sr isolate was dissolved in scintillation cocktail prior to counting of 89Sr and 90Sr in a single measurement with a low-level liquid scintillation spectrometer. Scintillation spectra and counting efficiencies for 89Sr, 90Sr and 90Y have been established. A simple spectral stripping technique may thus be applied to calculate activities from the spectra of real samples. Results of analyses of five 2-1 milk samples containing known activities of 89Sr and 90Sr were respectively 87.7 +/- 3.5% and 90.4 +/- 3.0% (+/- 1 S.D.) of the activities added. A correction factor for the Sr yield was not applied. The detection limit of 0.014 Bq for 90Sr samples free of 89Sr implies that the method is also useful for routine monitoring of 90Sr.

Sub-structure of synthetic casein micelles.

Casein micelles of different composition were synthesized in various ways and their sub-structure was investigated with the electron microscope by means of thin sections. Earlier studies of Schmidt et al. (1974) using the freeze-fracturing technique had shown no differences in the sub-structure of natural micelles and artificial micelles containing Ca and casein only. Contrary to these results our present studies showed that for the production of synthetic casein micelles with the same sub-structure as the natural ones it is necessary to add at least 2 other ions to the casein solution besides Ca2+: these are phosphate and citrate. The citrate ions play an important role in forming the material of the dark framework of the micelles visible in electron micrographs of unstained thin sections. This supports the hypothesis of Pyne & McGann (1960) that casein micelles contain a citrate apatite.