QUARTZ-CRYSTAL MICROBALANCE SETUP FOR FREQUENCY AND Q-FACTOR MEASUREMENTS IN GASEOUS AND LIQUID ENVIRONMENTS

An experimental setup has been constructed for simultaneous measuremen ts of the frequency, the absolute Q factor, and the amplitude of oscil lation of a quartz crystal microbalance (QCM). The technical solution allows operation in vacuum, air, or liquid. The crystal is driven at i ts resonant frequency by an oscillator that can be intermittently disc onnected causing the crystal oscillation amplitude to decay exponentia lly. From the recorded decay curve the absolute Q factor (calculated f rom the decay time constant), the frequency of the freely oscillating crystal, and the amplitude of oscillation are obtained. All measuremen ts are fully automated. One electrode of the QCM in our setup was conn ected to true ground which makes possible simultaneous electrochemistr y. The performance is illustrated by experiments in fluids of varying viscosity (gas and liquid) and by protein adsorption in situ. We found , in addition to the above results, that the amplitude of oscillation is not always directly proportional to the Q factor, as the commonly u sed theory states. This puts limitations on the customary use of the a mplitude of oscillation as a measure of the a factor. (C) 1995 America n Institute of Physics.