Uranium hexafluoride () was the only known compound of uranium sufficiently volatile to be used in the gaseous diffusion process. Before this could be done, the "Special Alloyed Materials (SAM) Laboratories" at Columbia University and the Kellex Corporation had to overcome formidable difficulties to develop a suitable barrier. Fortunately, fluorine consists of only a single isotope , so that the 1percent difference in molecular weights between and is due solely to the difference in weights of the uranium isotopes. For these reasons, was the only choice as a feedstock for the gaseous diffusion process. Uranium hexafluoride, a solid at room temperature, sublimes at at . Applying Graham's law to uranium hexafluoride: