A method of constructing an ion guide is disclosed comprising providing an elongated spine member and a plurality of plates. Each plate comprises an aperture therethrough for receiving the spine member and at least one electrode for use in guiding ions. The apertures of the plates are arranged around the spine member and the plates are arranged along the spine member. The plates are then locked in position on the spine member such that the plates are fixed axially with respect to the spine member and so that the electrodes of the plates are arranged so as to form an array of electrodes for use in guiding ions.