Gaseous diffusion had been used to separate isotopes before. Francis William Aston had used it to partially separate isotopes of neon in 1931, and Gustav Ludwig Hertz had improved on the method to almost completely separate neon by running it through a series of stages. In the United States, William D. Harkins had used it to separate chlorine. Kistiakowsky was familiar with the work of Charles G. Maier at the Bureau of Mines, who had also used the process to separate gases.