PHASE-FREQUENCY RELATIONSHIPS IN OSCILLATING QUARTZ MICROBALANCE ELECTRODES - DETERMINATION OF AN OPTIMAL OPERATING FREQUENCY FOR SOLUTION-PHASE MICROGRAVIMETRY

A systematic study of the effects of overtone harmonic, operating frequency, and electrode surface area on mass sensitivity of liquid-phase quartz-crystal microbalance (QCM) operation is presented. Although the Sauerbrey equation implies that sensitivity should increase as the square of the operating frequency, the effects of mechanical damping induced by operating the crystal under liquid media causes a loss of phase/mass sensitivity at higher nominal frequencies. The competing effects provide a optimum frequency at which to operate the crystal for maximum mass sensitivity under liquids. In the case of water, this optimum frequency is 12.4 MHz. As the phase of the complex impedance is used by most series resonance crystal maintaining circuits as an error signal in stabilizing frequency, progressively higher operating frequencies are not desirable for QCM operation under liquids. © 1995 American Institute of Physics.