Determination of 90Sr in the aftermath of a radiological event

High resolution inductively coupled plasma mass spectrometry (ICP-MS) for rapid determination of 90Sr was evaluated. The isobaric interference 90Zr was reduced either by using cold plasma conditions or with chemical separation. The chemical separation yielded both a better separation and a more stable plasma. However, tailing in the mass spectrum, caused by natural strontium in the sample matrix, sets the main limitation, which was shown in the analysis of a vegetation sample. The detection limit for a relatively simple matrix, such as tap water (300 mL), varied from 50 to 200 Bq/L, to be compared with the IAEA generic action level at 100 Bq/L for an emergency preparedness situation. A mathematical model which optimises detection level, ingrowth and counting time (total measurements time) for radiometric determination of 90Sr was developed. For a given number of samples, the model predicts detection limit as a function of total measurement time, as well as the time fractions to be spent on ingrowth and counting. For example, analysis of ten, 10 ml, milk or water samples require approximately 11 h total measurement time for obtaining a detection limit lower than the generic action level. Ten, 100 ml samples only require about 3 h to obtain the same detection limit. The main limitation in analysing 90Sr with ICP-MS comes from the fact that most sample matrices contain natural strontium in such amounts that there will be an abundance sensitivity problem, i.e. natural strontium on m/z=88 will tail into m/z=90. With the ICP-MS used in this study, Finnigan Element2, a general recommendation must be to perform a radiometric determination of 90Sr.