Using quartz crystal microbalances to simultaneously sense mass accumulation and solution properties

A lumped-element equivalent circuit model is presented that describes the near-resonance electrical characteristics of a quartz crystal microbalance (QCM) simultaneously loaded by a surface mass layer and a contacting liquid. The model was derived by solving the boundary-value problem for coupled mechanical displacement and electrical potential; this results in circuit elements that are explicitly related to physical properties of the quartz, the perturbing mass layer, and the contacting liquid. The effects of mass and/or liquid loading on the admittance-vs.-frequency behavior are predicted from the model and compared with experimental impedance analyzer measurements. Surface mass accumulation causes a simple translation in frequency of the resonance peak, while increasing the density-viscosity product of the contacting solution causes both a translation and a damping of the resonance peak. Using the model, changes in surface mass can be differentiated from changes in solution properties.<
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