As a result of his study of radioactive elements, Frederick Soddy concluded in 1913 that an element might occur in more than one form. The multiple forms of the element all have the same atomic number, but different atomic weights. Soddy referred to these forms as isotopes. Shortly after Soddy's work, Joseph J. Thomson and Francis Aston demonstrated that isotopes exist among stable elements as well as among radioactive ones.

The separation of isotopes has long presented a challenge to scientists. Because the isotopes of a single element all have the same electron structure, they have essentially the same chemical properties. The differences in their masses, due to varying numbers of neutrons in their nuclei, however, results in their having different physical properties. These differences in physical properties form the bases for the roughly half dozen different methods used for the separation of isotopes.

The mass spectrometer...