Calcium tungstate coprecipitation for removal of Sr interference with determination of Rb by ID-ICP-MS

A coprecipitation method using calcium tungstate was developed to remove 87Sr isobaric interference with 87Rb prior to measurement of Rb by ID-ICP-MS. Precipitation of calcium tungstate was obtained by adding Ca(NO3)2 solution and (NH4)2WO4 solution to the sample, where (NH4)2WO4 was added more than the stoichiometric proportion to precipitate Ca(NO3)2 completely and remove Sr effectively. Furthermore, in order to reduce matrix burden to the ICP-MS instrument, the residual (NH4)2WO4 was removed by adding conc. HNO3. Prior to the application, thorough purification of coprecipitant reagent was carried out to reduce the blank. The effectiveness of the present method was verified by analyzing two brown rice flour certified reference materials (CRMs), NIES CRM 10a and NIES CRM 10b. Finally, the present method was applied to the measurement of Rb in a white rice flour RM sample being developed by National Institute of Metrology of Japan.