Gas Chromatography Detectors - Some Less Common GC Detectors > The Ultrasound Whistle Detector > Page 87


The sensor consisted of a metal cylinder that acted as one electrode with a coaxial wire passing down the center that acted as the other. A 40 MHz radio frequency was applied across the electrodes and the DC potential that developed across them was fed via simple electronic circuit to a potentiometric recorder. The resistance capacity decoupling shown in their circuit appears hardly sufficient to achieve the removal of the AC signal in a satisfactory manner and consequently, the circuit shown in figure 50 may be only schematic. The column was connected directly to the sensor and the eluent passed through the annular channel between the central electrode and the sensor wall.

The response of the radio frequency discharge detector was reported as 106mV for a concentration change of 10-3 g/ml of methyl laureate. The noise level was reported to be 0.05 mV, which would give the minimum detectable concentration for a signal–to–noise ratio of 2 as about 6 x 10-10 g/ml. This detector had the advantage of operating at atmospheric pressure and so no vacuum system was required. The effect of temperature on the detector performance was not reported, nor was its linearity over a significant concentration range. This detector appears not to have been made commercially.


The Ultrasound Whistle Detector

The velocity of the propagation of sound through a gas depends on its density and, thus, the presence of a solute vapor in a gas changes the velocity of sound through it. This velocity change can be utilized as a basis for vapor detection in GC. The frequency of a whistle, consisting of an orifice which directs a stream of gas against a jet edge proximate to a resonant cavity, is related to the velocity of sound in the gas passing through it. A diagram of such a whistle is shown in figure 12. Nyborg et al. (38) showed that the frequency (fn) of the whistle could be described by the following equation.


where (n) is an integer,
(c) is the velocity of sound in the gas,
(L) is the cavity length,
and (e) is the end effect.