USING THE QUARTZ MICROBALANCE PRINCIPLE FOR SENSING MASS CHANGES AND DAMPING PROPERTIES

A brief summary of the most important development steps of a special c rystal sensing technique is presented. Then, a lumped-element equivale nt circuit model is introduced that describes the near-resonance elect rical characteristics of a quartz-crystal microbalance (QCM) simultane ously loaded by a surface mass layer and contacting analyte. This mode l predicts that surface mass accumulation causes a shift of the resona nce peak while increasing the density-viscosity product of the sensiti ve 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 qu artz crystal and various solvent vapours (analytes). Different polymer s, modified ethanolamines and compounds with long alkyl-chains are use d as coatings. Two of these coated sensors show an anomalous effect: t he exposure to solvent vapours results in an increase of frequency bec ause 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.