A microprocessor-based piezoelectric quartz microbalance system for compound-specific detection

The authors describe the implementation of a microprocessor-controlled piezoelectric quartz microbalance (PQM) system suitable for field deployment. The system has been designed to detect nanogram-level mass changes of sorbed compounds sensitive to the specific coating materials used. Miniaturized modular hybrid crystal clock oscillators were used as the sensing and reference elements. Five oscillator modules were used to construct a four-sensor array. The quartz crystals of four modules were exposed and coated with chemically specific compounds so that they would act as sorption detectors. These sensor elements were enclosed in a small flow-through gas chamber along with a fifth module used as a reference oscillator. The frequency of each sensor was separately mixed with that of the reference oscillator and multiplexed into the microprocessor, which is measured the difference frequencies. The temperature within a gas chamber was maintained constant by means of a PID (proportional-integral-derivative) controller. A Peltier device was used as the heating/cooling element of the gas chamber