Abstract: In an MS-MS time-of-flight mass spectrometer, a space focus of the ion source is defined by correction of the second order. If the geometrical and electric values of the ion source are suitably selected, the space focus may be such as to permit very good primary mass resolution when suitable secondary interaction methods are used. The secondary interaction at the space focus may be effected (a) by a focused, pulsed laser ray or other pulsed interaction methods that can be focused. (b) by a wire mesh consisting of very fine "line combs" engaging each other, to which voltage pulses can be applied, (c) by a combination of a) and/or b) with an electrostatically high, primary, fieldless drift path. The MS-MS time-of-flight mass spectrometer is operated using a reflector comprising a movable reflector end plate with adjustable potential, which enables primary ions to be eliminated from the spectrum without any loss in mass resolution.