STERIC FACTORS AFFECTING THE DISCRIMINATION OF ISOMERIC AND STRUCTURALLY RELATED OLEFIN GASES AND VAPORS WITH A REAGENT-COATED SURFACE ACOUSTIC-WAVE SENSOR

An investigation of steric factors affecting the olefin-olefin selectivity of a surface acoustic wave (SAW) sensor coated with reagents of the general formula trans-PtCl2(ethylene)-(substituted-pyridine) is described. Detection is based on the mass increase accompanying replacement of ethylene by other gas-phase olefins to form the corresponding olefin-substituted products. Selectivity depends on the relative reaction rates of the different olefins. Within series of structurally similar butenes, acrylates, and aromatic olefins, unusually high selectivity is observed for the less hindered olefins and complete discrimination of isomers is achieved in certain cases. Replacing pyridine by 2-methylpyridine and 2,6-dimethylpyridine in the reagent complex progressively reduces the sensor response. Sensitivities increase with increasing temperature and limits of detection ranging from about 2 to 70-mu-g/L are achieved with modest heating (30-40-degrees-C) using a 30-MHz SAW oscillator. Initial results with a 52-MHz sensor show a 3.4-fold increase in sensitivity compared to the 30-MHz sensor in rough agreement with theory.