In a secondary ion mass spectrograph, the electrical power supply voltages controlling and pertaining to the polarity of the target secondary ions are all switched between opposite polarities simultaneously while the deflection of the primary ion beam is automatically corrected for any error that may result from the polarity switch-over. This allows quick alternation of the polarity of target ions without loss of accuracy of the primary beam scan. The correction to the primary beam deflected may be predetermined under specific observing conditions or may be derived from theoretical considerations.