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

The authors describe the implementation of a portable, microprocessor-controlled, piezoelectric quartz microbalance (PQM) system suitable for remote and autonomous monitoring of air quality around hazardous waste sites or chemical spills. The system has been designed to detect nanogram-level mass change or sorbed compounds sensitive to the specific coating materials used. Miniaturized modular hybrid crystal clock oscillators were used to construct a four-sensor array. The quartz crystals of the 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 which was left sealed and was used as a reference oscillator. The frequency of each sensor was separately mixed with that of the reference oscillator and multiplexed into the microprocessor. The difference frequencies were measured by the microprocessor through a programmable timer/counter chip. The temperature within the gas chamber was maintained constant by means of a PID (proportional-integral-derivative) controller implemented in software. A Peltier device was used as a heating/cooling element of the gas chamber