Surface acoustic-wave piezoelectric crystal aerosol mass microbalance

Abstract

The feasibility of applying surface acoustic-wave (SAW) piezoelectric crystal technology to the measurement of subnanogram levels of particles and parts-per-million levels of sulfur dioxide has been demonstrated. Improvements necessary for implementing the development of a practical instrument for the real-time measurement of very low aerosol mass and ambient gases have been identified. Mass sensitivity comparisons of a 158-MHz SAW piezoelectric microbalance and a conventional 10-MHz quartz-crystal microbalance (QCM) showed that the SAW crystal was 266 times more sensitive. This is in good agreement with the theoretical value of 250. Frequency stability of a single SAW resonator was 6 parts in 108 over 1 min. Response to temperature change is found to be very linear over the range +30 to −30 °C. Response to pressure change is found to exhibit an aging effect whereby after one complete cycle of pressure variation from 1 atm to 70 mm Hg and back to 1 atm, the SAW response to pressure change decreases by an order of magnitude. A strong response to 15 ppm SO2 has been demonstrated on a chemically coated SAW crystal. With improvements, SO2 measurements at the ppb level should be achievable.