In this work, the activity concentrations of 210Po, 234U, 238U and 228Ra radioisotopes are determined in mineral waters by alpha-particle spectrometry and liquid scintillation counting, respectively. Strontium-90 in wild berries samples is determined in view of certification of this material as reference material and its activity concentration is determined by liquid scintillation counting. A new method was developed and evaluated for the sequential determination of uranium and polonium radioisotopes using two different pre-concentration methods and two separation procedures. The advantage of this new method is that polonium and uranium isotopes can be determined simultaneously in the same sample. The activity concentration of 228Ra was determined via its daughter 228Ac with liquid scintillation counting, and with a newly developed method for its determination via its granddaughter 228Th and alpha-particle spectrometry. The results of activity concentration of 228Ra obtained by the liquid scintillation method were used for the establishment of the reference value for an interlaboratory comparison of IRMM. The determination of 90Sr in the newly developed certified reference material of wild berries constitutes the last part of the thesis, with emphasis on two steps, the long term stability study and the characterisation of the property value.