The working principle of potentiometric sensors is based of extremely small amounts of ions entering or leaving an electrode surface. When a potentiometric sensor shows a stable and constant potential the so-called potential determining ion has established the highest exchange current density compared to other ions. Since the exchange current densities are in the micro amps per qcm range the net amount of charge carriers responsible for the selective potential are often less than 10 to minus 12 g-moles. This is far to less for a QCM to be detected. The latter works better in immune-biosensors, when larger amounts of a substance are bound to a surface increasing their mass.