Gas Chromatography Detectors - The Flame Ionization detector > Electrode Configurations > Page 38
The use of high voltages in conjunction with the very small ionic currents require that all connections to the jet or electrode must be well insulated and electrically screened. In addition, the screening and insulating materials must be stable at the elevated temperature of the detector oven. In order to accommodate the high temperatures that exist at the jet-tip, the jet is usually constructed of a metal that is not easily oxidized such as stainless steel, platinum or platinum/rhodium.
The associated electronics of the FID sensor consist of a high voltage power supply and a high impedance amplifier. The jet and electrode can be connected to the power supply and amplifier in basically two different configurations; the two alternatives are shown in figure 17. The floating jet configuration is the most commonly used and is shown on the left in figure 17. One side of the high voltage power supply is earthed (grounded) and the other is taken to the cylindrical electrode. A high resistance is situated between the jet and earth. This resistance is changed to adjust the sensitivity of the detector but at the maximum sensitivity may be as great as 1000 M.
Figure 17. Electrode Configurations