An ion guide includes multiple stages. An electric field within each stage guides ions along a guide axis. Within each stage, amplitude and frequency, and resolving potential of the electric field may be independently varied. The geometry of the rods maintains a similarly shaped field from stage to stage, allowing efficient guidance of the ions along the axis. In particular, each rod segment of the i.sup.th of stage has a cross sectional radius r.sub.i, and a central axis located a distance R.sub.i+r.sub.i from the guide axis. The ratio r.sub.i/R.sub.i and is substantially constant along the guide axis, thereby preserving the shape of the field.