A mass spectrometer is disclosed comprising a quadrupole rod set ion trap wherein a potential field is created at the exit of the ion trap which decreases with increasing radius in one radial direction. Ions within the ion trap are mass selectively excited in a radial direction. Ions which have been excited in the radial direction experience a potential field which no longer confines the ions axially within the ion trap but which instead acts to extract the ions and hence causes the ions to be ejected axially from the ion trap.