b. Extraction Step. to the residue in the flask. Add 5 ml-of saturated aluminum nitrate solution Add 5.00 mi of reagent-grade ethyl acetate and thoroughly mix the solutions for about a minute using an electrically driven stirrer or a rod extractor. Allow the layers to separate. Recovery of the uranium in this step is known to be 90% or better and: removal of quenching elements is known to be reliable. 15 c. Fusion Step. FPipet 50- to 250-microliter portions of the ethyl Choose volumes that will yield an acetate layer to clean platinum dishes. estimated 0.05 to 0.1 pg of uranium. Place the dishes on graphite blocks and carefully evaporate to dryness. Add about 300 mg of the flux to each dish. A 2-cc open-ended hypo- dermic syringe is used to measure out and transfer 0.4 cc of flux in each case. Transfer the dishes to a Fischer burner and fuse at the lowest temperature possible. Keep the mixtures molten for 30 seconds and then remove and allow them to cool for 15 minutes before measuring the fluorescence. The essential factor in the fusion step is uniformity of treatment of standards and the unknown ins2far as possible, Minimum temperature and a short molten time are important to limit the dissolution of platinum in the flux which acts as a quenching element. d. Uranium Standards and Blanks. One liter of a standard solution of uranium is prepared by accurately weighing about five grams of pure uranium metal and dissolving in a mixture of hydrochloric and nitric acids. A working standard in 2M nitric acid is made by diluting this solution exactly by a factor of one thousand. Aliquots of this solution are then used to obtain known amounts of uranium for the preparation of standard samples. To correct for possible inclusion of uranium from the papers, reagents, vessels, and to some extent accidental contaminations a blank must be run. A clean filter paper is dissolved using the same reagents in the same amounts as those employed in dissolving the sample. Aliquots of this solu- tion are carried through all the steps of the uranium determination and the level of their fluorescence is measured on the fluorometer. Accurately known amounts of uranium, about 2 pg, are added to aliquots of the above blank solution. These standards are carried through 7 ° J. A. S, Adams and W. J. Maeck, Anal. Chem. 26, 1635 (1954). -47 .~