VAPORIZATION AND ATOMIZATION OF URANIUM IN A GRAPHITE TUBE ELECTROTHERMAL VAPORIZER - A MECHANISTIC STUDY USING ELECTROTHERMAL VAPORIZATION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY AND GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY

The mechanism of vaporization and atomization of U in a graphite tube electrothermal vaporizer was studied using graphite furnace atomic absorption spectrometry (GFAAS) and electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Graphite furnace AAS studies indicate U atoms are formed at temperatures above 2400 degrees C. Using ETV-ICP-MS, an appearance temperature of 1100 degrees C was obtained indicating that some U vaporizes as U oxide. Although U carbides form at temperatures above 2000 degrees C, ETV-ICP-MS studies show that they do not vaporize until 2600 degrees C. In the temperature range between 2200 degrees C and 2600 degrees C, U atoms in GFAAS are likely formed by thermal dissociation of U oxide, whereas at higher temperatures, U atoms are formed via thermal dissociation of U carbide. The origin of U signal suppression in ETV-ICP-MS by NaCl was also investigated. At temperatures above 2000 degrees C, signal suppression may be caused by the accelerated rate of formation of carbide species while at temperatures below 2000 degrees C, the presence of NaCl may cause intercalation of the U in the graphite layers resulting in partial retention of U during the vaporization step. The use of 0.3% freon-23 (CHF3) mixed with the argon carrier gas was effective in preventing the intercalation of U in graphite and U carbide formation at 2700 degrees C.