An ion guide is disclosed comprising a plurality of electrodes. A first device is arranged and adapted to apply a RF voltage to at least some of the electrodes in order to form, in use, a pseudo-potential well which acts to confine ions in a first direction within the ion guide. A second device is arranged and adapted to apply a DC voltage to at least some of the electrodes in order to form, in use, a DC potential well which acts to confine ions in a second direction within the ion guide. A third device is arranged and adapted to cause ions having desired or undesired mass to charge ratios to be mass to charge ratio selectively ejected from the ion guide in the second direction.