DETERMINATION OF TRACES OF NP-237 IN ENRICHED URANIUM SOLUTIONS USING INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY

A procedure using inductively coupled plasma mass spectrometry (ICP-MS) has been developed for determining traces of Np-237 in isotopically enriched solutions of U. The chemical treatment of the samples consists of: extracting Np-237 in a 1 mol dm-3 solutions of nitric acid using 0.5 mol dm-3 thenoyltrifluoroacetone in xylene, leaving 99% of the U matrix in the aqueous phase; re-extracting Np-237 in 10 mol dm-3 nitric acid; heating the extracted solution to white fumes in order to remove the excess of acid; and diluting the resulting solution with 2% HNO3. Optimization of the analytical parameters involved suppressing the spectral overlap between Np-237 and U-238. By applying an additive continuous tension to the quadrupole on m/z 237 only, the interfering part of the U-238 peak is eliminated and good resolution can be achieved between m/z 237 and 238, if the concentration of U is less than 100 mg dm-3. The operating conditions adopted result from a compromise between resolution and sensitivity. The suppression of the analyte signal, arising from U at a high concentration, is corrected for by adding an internal standard to the calibration and measuring solutions. The detection limit is about 0.005 mug of Np per gram of U, for an initial sample of 1 g of U and an additive continuous tension of 7.5 V. The analytical performance of ICP-MS is compared with that of alpha-spectrometry The radiochemical determination of Np-237 involves a tedious pre-treatment and measurement procedure. In contrast, ICP-MS appears to be a good alternative to alpha-spectrometry (for the determination of long-lived radionuclides) because of its simplicity, accuracy, excellent detection limits and rapidity of analysis.