Using the quartz microbalance principle for sensing mass changes and damping properties

A brief summary of the most important development steps of a special crystal sensing technique is presented. Then, a lumped-element equivalent circuit model is introduced that describes the near-resonance electrical characteristics of a quartz-crystal microbalance (QCM) simultaneously loaded by a surface mass layer and contacting analyte. This model predicts that surface mass accumulation causes a shift of the resonance peak while increasing the density-viscosity product of the sensitive layer. These two effects cause both a translation and a decrease of the resonance peak. Experimental results are given which demonstrate interactions between sensitive materials (adsorbates) coated on the quartz crystal and various solvent vapours (analytes). Different polymers, modified ethanolamines and compounds with long alkyl-chains are used as coatings. Two of these coated sensors show an anomalous effect: the exposure to solvent vapours results in an increase of frequency because of the effective mass decrease. The apparent discrepancy implies that the physical properties of the film must change, requiring a more detailed characterization of the coated quartz.