Gas Chromatography Detectors - Some Less Common GC Detectors > The Dielectric Constant Detector > Page 90

 

The Dielectric Constant Detector

In1961Winefordneretal.(40)described a GC detector that functioned on the change in dielectric constant of the carrier gas when a vapor or another gas was present. Like the katharometer, the detector responded to most gases and vapors providing a suitable carrier gas was chosen. The sensor took the form of a variable capacitor mounted in a special cell which allowed the column eluent to pass between its plates. The capacitor was made to constitute part of the "tank" circuit of a Clapp oscillator. The output of the oscillator was allowed to beat against a reference oscillator and the beat frequency was takenasthesensor signal. The detectorwas reported to respond linearly to changes in vapor concentration. However, the device was found to be no more sensitive than the katharometer and considerably more complex.

In 1965 Winefordner et al. (41) developed the detector further. They employed virtually the same physical system but in this case they used a miniature coaxial type sensor with very small spacing between the internal and external cylindrical conductors (0.005 in.). Significantly improved sensitivities of 10-10 g/ml were reported for the detection of oxygen, nitrogen, hydrogen, carbon dioxide, carbon monoxide, methane, nitrogen dioxide, nitrous oxide and sulfur dioxide. In addition, the authors claimed by employing solid state electronics in place of the thermionic tubes the electronic noise the sensitivity could be increased by one or two orders of magnitude. Subsequently Williams and Winefordner claimed a "nearly" linear response for the detector over a concentration range in excess of 4 orders of magnitude for the gases ethylene, ethane, propane and ammonia.