Although this improved the performance of the detector and reduced the necessary sample size, the detecting system was never made commercially. Even after modification, its sensitivity was relatively poor and despite it being an absolute detecting system, it placed too many restrictions on the operation of the chromatograph and the samples that could be chromatographed to be generally useful.
The Surface Potential Detector
The surface potential detector was developed by Griffiths and Phillips [47,48] in the early 1950s and consisted of a cell containing two parallel metal plates between which flowed the column eluent. One plate was mechanically attached to an oscillator that vibrated the plate at about 10 kHz. If the plates are identical, the surface charge on each plate is the same and so no potential is induced into the second plate by the vibrating plate. If however the surfaces are dissimilar, then the surface charge on each plate will differ and the vibrating plate will induce a potential on the other plate. A diagram of the detector is shown in figure 62.
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
