The QCMD-E4 and PGSTAT 302 Potentiostat has its foundations in the traditional QCM technology where mass added to the electrodes of a quartz crystal resonator is monitored by the change in resonance frequency. The QCM technology comprises specially cut quartz sandwiched between electrodes such as gold and aluminum. The piezoelectric nature of the quartz means that it resonates at a particular frequency under the application of an AC voltage across the electrodes. For a certain amount of added mass, the frequency, f, decreases proportionally, as described by Sauerbrey. The Sauerbrey relation is however only valid for rigid films, and is hence not sufficient for loosely structured (visco-elastic) adsorbents like water rich protein films. The introduction of the QCM-D has allowed for the monitoring of an additional factor, D, the damping of dissipation which gives information on the structure of the film adhered to the sensor. The dissipation factor of a quartz oscillator is measured by recording how the oscillation decays after the oscillator has been excited into oscillation.