Surface acoustic wave probes for chemical analysis

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Virginia Polytechnic Institute and State University

Surface Acoustic Wave delay lines have been used as probes for chemical analysis. The interaction between matter adjacent to the SAW device surface and the propagating Raleigh wave caused measurable changes in the amplitude, phase and resonant frequency of the wave. The effectiveness of various electronic detection schemes was evaluated along with the response of the device to changes in pressure and temperature.

A lithium niobate SAW device was used as a detector for gas chromatography. Frequency shifts of a SAW oscillator provided the highest sensitivity to compounds eluting from the G.C. column. Sensitivity and specificity of the detector to polar and non-polar organic compounds was greatly enhanced by thin chemical coatings applied to the detector surface. Submicrogram quantities of material were easily detected. Linearity and dynamic range of the detection system was poor. Numerous refinements remain to be made which could significantly improve performance.

Thermomechanical analysis of thin polymer films were accomplished using a 32 MHz quartz delay line. Very large wave amplitude shifts were observed as the polymer reached its glass transition temperature. Tg measurements were performed on samples clamped to the surface and cast on the surface. Agreement with low frequency dynamic mechanical measurements was good for the clamped specimens, indicating the absence of wave coupling. Specimens cast on the surface experienced large Tg shifts and therefore were coupled to the surface wave. More subtle transitions were also detected. A crystalline transition around room temperature in a TEFLON film clamped to the SAW device provided an easily observed shift in SAW amplitude. Explanations of this behavior have been proposed. The SAW device also provided an ideal vehicle for examining the behavior of thin photoresist films on the surface. Information on solvent evaporation processes and photo-induced crosslinking rate was obtained. The attractive features of the device for polymer thermomechanical analysis include low cost, ruggedness, high sensitivity and ease of use.