Gas Chromatography Detectors - Closing Notes > Page 95


Both plates were constructed of the same metal but one plate was coated with a monolayer of a suitable substance that would absorb any vapors present in the column eluent. The absorbing layer caused the charge on the two plates to be dissimilar and thus a potential appeared acrossthetwoplates which was balanced out by the bias potentiometer. When a solute vapor passes through the detector, some is distributed into the absorbent layer, changing the surface charge and thus inducing a change in potential between the electrodes. This produces an AC signal voltage that can then amplified, rectified and the output passed to a recorder (or to a data acquisition system). The signals provided by the detector sensor could be as great as several hundred millivolts.

The sensitivity of the detector was claimed to be similar to that of the katharometer (i.e. about 10-6 g/ml). Its response was partly determined by the distribution coefficient of the solute vapor between the carrier gas and the absorbing layer (and thus the chemical characteristic of the coating) as well as the chemical nature of the solute itself. As a consequence, the response varied considerably between different solutes. Within a given homologous series, however, the response increased with the molecular weight of the solute, but this was probably merely a reflection of the increase in the value of the distribution coefficient with molecular weight. Although an interesting alternative method of detection, this detector has been little used in GC and is not commercially available.

Closing Notes

Developments in the technique of gas chromatography is now relatively slow and as an analytical technique it might be aid it has reached a "steady state"; this is also true for the development of GC detectors. Very few GC detectors have been commercially introduced and become popular over the last decade and the four detectors, the FID, ECD, NPD and the katharometer are still the most commonly used as they were 20 years ago. In fact, they are even more established, and are employed in over 95% of all GC applications.