The invention relates to a method and device for the monitoring of a medical microsample in the flow measuring cell of an analyzer with regard to position and absence of bubbles by means of an alternating voltage applied to the measuring cell, the measuring cell being provided with a multitude of electrode systems placed one behind the other, each system comprising a number of single electrodes for measuring a substance contained in the microsample by means of a measurement voltage which essentially is a DC voltage. To monitor the exact position of the microsample and/or to detect air bubbles in the area of each electrode system, the alternating voltage and the measurement voltage are simultaneously and directly applied to the single electrodes of the corresponding electrode system, and the measured AC component respectively the measured impedance gives a measure for the position of the microsample and the absence of bubbles.