Determination of Neptunium and Plutonium in the Presence of High Concentrations of Uranium by Ion Chromatography–Inductively Coupled Plasma Mass Spectrometry

Ion chromatography (IC) coupled with ICP-MS was used to separate Np, Pu and U, overcoming the isobaric interferences present when the isotopic composition has to be determined and avoiding the influence of the tail of the 238 U peak on the 237 Np and 239 Pu peaks in samples with a high U content. The optimization of the parameters was carried out with a standard solution containing Np, U and Pu (ratio 1:1:1) at the 100 ng ml -1 concentration level. The precision and accuracy of the method were also evaluated using a sample containing 1 ng ml -1 of Np and Pu and 100 ng ml -1 of U. A 200 µl aliquot of the sample (previously treated with silver oxide in order to obtain the elements in a single oxidation state) was injected and, after passing through the separation column, the effluent was passed directly to the nebulizer. Different mobile phases and oxidizing/reducing agents were tested. The use of a high-capacity cation-exchange column (CS10), silver oxide as oxidizing agent and 40 mm 2,3-diaminopropionic acid monohydronitrate in 0.6 m HNO 3 as eluent in isocratic mode provided good resolution and recovery of the elements studied (96–98%). The precision of the method is 2.7, 1.1 and 1.7% for Np:U, Np:Pu and U:Pu, respectively, for a solution containing Np, Pu and U in a ratio of 1:1:1. Isotope dilution analysis applying IC separation was performed for the determination of Pu. The method of standard additions was used for the determination of Np. The agreement between the IC–ICP-MS results and the calculated values was within 15% for most isotopes.