A band gap mass filter for separating particles of mass (M₁) from particles of mass (M₂) in a multi-species plasma includes a chamber defining an axis. Coils around the chamber generate an axially aligned magnetic field defined (B=B₀+B₁ sin t), with an antenna generating the sinusoidal component (B₁ sin t) to induce an azimuthal electric field (E) in the chamber. The resultant crossed electric and magnetic fields place particles M₂ on unconfined orbits for collection inside the chamber, and pass the particles M₁ through said chamber for separation from the particles M₂. Unconfined orbits for particles M₂ are determined according to an - plot ##EQU1##

where ₀ is the cyclotron frequency for particles with mass/charge ratio M, and wherein ₀=B₀/M and ₁=B₁/M.