DETERMINATION OF ULTRATRACE AMOUNTS OF URANIUM AND THORIUM IN YTTRIUM BY THE ION-EXCHANGE SEPARATION/ICP-MS

A method has been developed for the 0.01 ng/g level determination of uranium and thorium in yttrium by electrothermal vaporization(ETV)/ICP-MS. An ion-exchange technique was applied for the separation of uranium and thorium from yttrium and other elements in the sample. Although uranium and thorium were adsorbed on an anion exchange resin, yttrium was not adsorbed in a 7 M nitric acid solution. Uranium and thorium were eluted with a 1 M hydrofluoric acid-0.1 M hydrochloric acid mixture. The eluted solution was evaporated to dryness and dissolved again with 1M hydrochloric acid. Uranium and thorium in the solution were further separated from traces of concomitant elements by a cation-exchange resin. Uranium was eluted with 2 M hydrochloric acid. After yttrium, lanthanum, cerium, praseodymium and neodymium were eluted with 4M nitric acid and 4M hydrochloric acid, and thorium was eluted with a 1 M hydrofluoric acid-0.1 M. hydrochloric acid mixture. The eluted solutions of uranium and thorium were combined and evaporated to dryness. The residue was dissolved;with 1M hydrochloric acid. Uranium and thorium were determined by ETV/ICP-MS using tungsten and molybdenum boats, respectively, because the tungsten boat contained high level of thorium and the molybdenum boat contained uranium. The determination limits of uranium and thorium were 0.003 and 0.005 ng/g, respectively.