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

Abstract

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.