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The Disc Detector

The disc detector was originally described by Dubsky (46) who used a rotating gauze disc as the carrier. A diagram of the device is shown in figure 47. The rotating disc has a perimeter made of wire gauze and the column exit is situated just above the gauze so that the eluent flows through the gauze coating it with a film of mobile phase. The excess of mobile phase is collected in a container situated below the disc. An infrared lamp, or some other appropriate heater, is placed a little ahead of the point of coating, in the direction of rotation of the disc which evaporates the solvent leaving the solute coated on the gauze.

Figure 47. The Disc Detector

The FID is placed diametrically opposite to the point of coating and the flame jet is situated beneath the gauze in such a manner that the flame itself is in contact with the gauze.

The FID electrodes are placed just above the gauze, directly over the flame. The ion current is collected by the electrodes and amplified in the usual way and fed to a recorder or data acquisition system. This arrangement is simple compared with the conventional wire transport detector but although a tenfold increase in sensitivity is claimed, this is difficult to confirm from the published data given. Szakusito and Robinson (47) reported that the metal gauze disc carrier was a source of excessive noise. Specifically, significant  "spikes" were caused by local concentrations of solute accumulating at the intersections of the wire mesh during evaporation. These authors employed an alumina disc, 4.5 in diameter, with the edge tapered to 0.25 mm thick. The thin edge was used for coating and detection. It appeared that there a significant reduction in noise, but again, sensitivities were not given in terms that would allow comparison with other detectors. It appeared that in continual use, the pores of the alumina could become blocked with residue from incompletely combusted solutes or mobile phase components and so the life of the alumina disc may well be limited. The disc does appear to be a simpler transport system than the wire or chain but its reliability and sensitivity remain to be established.