Gas Chromatography Detectors - The Response Mechanism of the FID > Page 41

Once electron/ion pair production is initiated the recombination starts to take place. The longer the ions take to reach the electrode the more recombination takes place. Thus, the greater the distance between the electrodes and/or the lower the voltage, the greater the recombination. This is substantiated by the curves obtained by Ongkiehong: the plateau is reached at a lower voltage when the electrodes are closer together. It is seen that the plateau level is the same for both electrode conditions and it is assumed that on the plateau, all ions and electrons being produced in the flame are collected. In practice the applied voltage would be adjusted to suit the electrode distance to ensure that the detector operates under conditions where all electrons and ions are collected viz. on the plateau shown in figure 18.

Desty et al. (16) showed that the performance of the detector was relatively insensitive to electrode distance and hydrogen flow providing the voltage was adjusted to ensure that the detector worked on the current plateau.



Solutes: 1, n-hexane; 2, benzene; 3, cyclohexane; 4, 2,2,3-trimethylbutane; 5, n-heptane; 6, methylcyclohexane; 7, toluene; 8, 2,2,4-trimethylpentane; 9, n-octane; 10, ethylcyclohexane; 11, ethylbenzene.

Figure 19. Response oftheFID to Different Hydrocarbons