Sample Dissolution and Purification for ²³⁰Th-²³⁴U Age-Dating

This is the standard operating procedure used by the Isotope Ratio Mass Spectrometry Group of the Chemical Sciences Division at LLNL for the preparation of a sample of uranium oxide or uranium metal for ²³⁰Th-²³⁴U age-dating. The method described here includes the dissolution of a sample of uranium oxide or uranium metal, preparation of a secondary dilution, spiking of separate aliquots for uranium and thorium isotope dilution measurements, and purification of uranium and thorium aliquots for mass spectrometry. This SOP may be applied to uranium samples of unknown purity as in a nuclear forensic investigation, and also to well-characterized samples such as, for example, U₃O₈ and U-metal certified reference materials. The sample of uranium is transferred to a quartz or PFA vial, concentrated nitric acid is added and the sample is heated on a hotplate at approximately 100°C for several hours until it dissolves. The sample solution is diluted with water to make the solution approximately 4 M HNO₃ and hydrofluoric acid is added to make it 0.05 M HF. A secondary dilution of the primary uranium solution is prepared. Separate aliquots for uranium and thorium isotope dilution measurements are taken and spiked with ²³³U and ²²⁹Th, respectively. The spiked aliquot for uranium isotope dilution analysis is purified using EiChrom UTEVA resin. The spiked aliquot for thorium isotope dilution analysis is purified by, first, a 1.8 mL AG1x8 resin bed in 9 M HCl on which U adsorbs and Th passes through; second, adsorbing Th on a 1 mL AG1x8 resin bed in 8 M HNO₃ and then eluting it with 9 M HCl followed by 0.1 M HCl + 0.005 M HF; and third, by passing the Th through a final 1.0 mL AG1x8 resin bed in 9 M HCl. The mass spectrometry is performed using the procedure 'Th and U Mass Spectrometry for ²³⁰Th-²³⁴U Age Dating'.. Authors: Williams, R. W. [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)]; Gaffney, A. M. [Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)]. DOE Contract: W-7405-ENG-48; AC52-07NA27344. Subjects: 38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY; AGE ESTIMATION; DILUTION; DISSOLUTION; HYDROFLUORIC ACID; ISOTOPE RATIO; LAWRENCE LIVERMORE NATIONAL LABORATORY; MASS SPECTROSCOPY; NITRIC ACID; PURIFICATION; QUARTZ; RESINS; THORIUM; THORIUM ISOTOPES; URANIUM; URANIUM 228; URANIUM ISOTOPES; URANIUM OXIDES; WATER