Gas Chromatography Detectors - Some Less Common GC Detectors > The Piezoelectric Adsorption Detector > Page 91

The Piezoelectric Adsorption Detector

This detector is based on the fact that the frequency output from piezoelectric material is influenced by the weight of the coatings or layers on its surface. This phenomenon has been used for many years to measure trace concentrations of water vapor in a gas and, in addition xylene vapor in air has been detected by this means at concentrations as low as 10-6 g/ml. This method of detection was first introduced as a GC detecting system by King (42).

The detector consists of a quartz crystal (coated with a high boiling liquid) that is appropriately situated in an electronic circuit that causes it to oscillate at its natural frequency. The oscillation frequency is continuously monitored by a separate circuit. As material eluted from the column is absorbed by the coating, the weight of the crystal plus coating changes and the natural frequency falls. The relationship between oscillation frequency (f) and weight (w) absorbed is given by the following equation:


where (fo) is the natural frequency of the coated crystal,
and (A) is the total area of the coated crystal surface.



It follows that a standard crystal with a natural frequency of 9 MHz and a surface area of about one square centimeter will manifest a change in frequency of about 200 Hz for each microgram of adsorbed solute. Now frequency changes can be measured to within 0.1 Hz with normal equipment; consequently a change in mass adsorbed of 0.2 ng (10-9g) should be detectable. It would appear that this type of device should be very sensitive but, so far, it has not been made available commercially, at least, not as a GC detector.