Detection of unknown daughter ions using a mass spectrometer in which two mass spectrometric units are coupled together. The spectrometric unit in the front stage has either an electric field or superimposed fields. The spectrometric unit in the rear stage has superimposed fields. The voltage Vdx.sub.2 necessary to produce the electric field in the front stage and the voltage Vdx.sub.1 necessary to produce the electric field of the superimposed fields in the rear stage when daughter ions having known mass and energy are detected are found. Further, the voltage Vdx.sub.2 ' necessary to produce the electric field in the front stage and the voltage Vdx.sub.1 ' necessary to produce the electric field of the superimposed fields in the rear stage when unknown ions are detected are found. Both the mass and the energy of the unknown ions can be determined from these four voltages.