Identification and quantification of aqueous aromatic hydrocarbons using SH-surface acoustic wave sensors

A need exists for compact sensor systems capable of in situ monitoring of groundwater for accidental releases of fuel and oil. The work reported here addresses this need, using shear horizontal surface acoustic wave (SH-SAW) sensors, which function effectively in liquid environments. To achieve enhanced sensitivity and partial selectivity for hydrocarbons, the devices are coated with thin chemically sensitive polymer films. Various polymer materials are investigated with the goal of identifying a set of coatings suitable for a sensor array. The system is tested with compounds indicative of fuel and oil releases, in particular, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), in the low milligrams/liters to high micrograms/liters concentration range. Particular emphasis is placed on detection of benzene, a known carcinogen. It was observed that within the above concentration range, responses to multiple analytes in a mixture are additive, and there is a characteristic response time for each coating/analyte pair, which is largely independent of concentration. With the use of both the steady-state and transient-response information of SH-SAW sensor devices coated with three different polymer materials, poly(ethyl acrylate), poly(epichlorohydrin), and poly(isobutylene), a response pattern was obtained for benzene that is easily distinguishable from those of the other BTEX compounds. The time courses of the responses to binary analyte mixtures were modeled accurately using dual-exponential fits, yielding a characteristic concentration-independent time constant for each analyte/coating pair. Benzene concentration was quantified in the aqueous phase in the presence of the other BTEX compounds.