Abstract: An embodiment of the present invention provides an RF ion guide having four elongated electrodes arranged in parallel around the axial centerline. Each electrode is generally L-shaped in cross section, having first and second inner surfaces directed toward the interior of the ion guide. The first and second surfaces extend along axis that are transverse and preferably approximately perpendicular to one another. RF voltages of equal amplitude but opposite phases are applied to opposed pairs of electrodes, in the manner known in the art, to generate an RF field to radially confine ions and focus them to the centerline. Because the resultant RF field more closely approximates a quadrupolar field, relative to the field generated within a flatapole, better performance may be achieved in terms of improved transmission efficiencies and/or less mass discrimination.