Determination of 90Sr at ultratrace levels in urine by ICP-MS

Sr-90 appears as a radionuclide in the decay series of nuclear fission and can therefore be found in nuclear waste or released by nuclear accidents. Current methods for the detection of this radionuclide are time consuming and may be prone to a large variety of interferents. In this work, inductively coupled plasma mass spectrometry is explored for the determination of Sr-90 in the presence of stable zirconium in urine. Specific techniques are investigated to remove this as well as other contributions to the background at m/z = 90. A quadrupole ICP-MS equipped with a hexapole collision cell is first explored (final LOD = 2 ng L-1 for water samples), however, the desired limit of detection for Sr-90 in urine is quite low (0.02 pg L-1). The performance of a double-focusing sector field ICP mass spectrometer (ICP-SFMS) is further investigated, which allows measurement of Sr-90 at the ultratrace level. Other potential interferences were investigated and instrumental detection limits are calculated as 3 pg L-1 for water samples. Final parameters include the use of a cool plasma and medium mass resolution in ICP-SFMS. The method is applied to the analysis of Sr-90 extracted from urine using a crown ether extraction resin and concentrated (enrichment factor: 200); high levels of natural strontium in the separated fraction (of about 1 mug mL(-1)) equate to higher detection limits (80 pg L-1) due to Sr-88(+) at m/z = 90 and the relatively low abundance sensitivity of ICP-SFMS at medium mass resolution of 6 x 10(-7). This detection limit in the separated fraction corresponds to the detection limit of 0.4 pg L-1 in the original urine sample. The recovery of Sr-90, determined with the developed analytical method in spiked urine samples, was in the range of 82-86%.