An apparatus for separating ions includes an electrode arrangement having a length extending between first and second ends. The first end is configured to introduce a beam of ions into an ion transmission space of the arrangement. An electronic controller applies an RF potential and a DC potential to an electrode of the electrode arrangement, for generating a ponderomotive RF electric field and a mass-independent DC electric field. The application of the potentials is controlled such that a ratio of the strength of the ponderomotive RF electric field to the strength of the mass-independent DC electric field varies along the length of the electrode arrangement. The generated electric field supports extraction of ions having different m/z values at respective different positions along the length of the electrode arrangement. Ions are extracted in one of increasing and decreasing sequential order of m/z ratio with increasing distance from the first end.