Determination of traces of neptunium-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 $^{237}$Np in isotopically enriched solutions of U. The chemical treatment of the samples consists of: extracting $^{237}$Np 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 $^{237}$Np 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% HN0$_3$. Optimization of the analytical parameters involved suppressing the spectral overlap between $^{237}$Np and$^{238}$U. By applying an additive continuous tension to the quadrupole on m/z 237 only, the interfering part of the $^{238}$U 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 $\mu$g 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 $^{237}$Np 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.