New method of vapour discrimination using the Thickness Shear Mode (TSM) resonator

Abstract: The Impedance analysis technique complemented with curve fitting software was used to monitor changes in film properties of Thickness Shear Mode (TSM) resonator on vapour exposure. The approach demonstrates how sensor selectivity can be achieved through unique changes in film viscosity caused by organic vapour adsorption. Keywords: Impedance Analysis, QCM, TSM resonator, BVD model Introduction The TSM resonator is now widely used in many sensing applications. In most instances the TSM resonator is configured as a gravimetric mass sensor often referred to as a Quartz Crystal Microbalance (QCM), where the crystal works as the frequency-determining element in an electrical oscillator circuit. In chemical sensing applications the QCM is coated with a chemically active coating which absorbs the target analyte. The additional absorbed mass is reflected in the resonant frequency of the crystal and can therefore be detected [1]. This method is however limited as mass loading is only measured without considering the effect of characteristic properties of analytes. An improved method involves measuring the electrical characteristics (impedance/admittance) of the device over a range of frequencies near resonance. The obtained spectra are subsequently fitted to an equivalent circuit model as shown in Fig. 1, with each element representing a physical property of the crystal. R represents the energy loss arising from viscous effects and internal friction, L the initial mass/motional inertia of the system, C the mechanical elasticity of the quartz. C