Abstract: The apparatus and the method are for separating pulsed ions by mass as the pulsed ions are guided along a course. Each of the pulsed ions has a mass m within a range m.sub.min to m.sub.max, a speed v and substantially a same energy E, where E=1/2mv.sup.2. The pulsed ions pass a point P at a time T0. The apparatus comprises a guiding device for guiding pulsed ions along a closed circuit path; an insertion device having an insertion input for receiving ions, an insertion output for inserting ions deflected from the insertion input into the closed circuit path and a control gate for either activating or deactivating the insertion device; an extraction device having an extraction input for receiving ions guided along the closed circuit path, an extraction output for extracting ions out of the closed circuit path and a control gate for either activating or deactivating the extraction device; and a controller for controlling operation of the insertion and extraction devices.