The Flame Ionization Detector (FID)

The FID, which was invented by Harley and Pretorious (36) and separately, and almost simultaneously by McWilliams and Dewer (37), was a natural evolution of the Heat of Combustion Detector developed by Scott (38). A diagram of the basic FID is shown in figure 61.

Figure 61. The Flame Ionization Detector

The FID detector employs a stream of hydrogen as the combustion gas into which passes the column eluent (which may be helium, hydrogen or any other appropriate gas). The mixture of hydrogen and the column eluent is then burnt at a small jet. The jet is situated inside a cylindrical electrode and a potential of a few hundred volts is applied between the jet and the electrode. When a carbon containing solute is burnt in the jet, the electron/ion pairs that are formed are collected at the jet and cylindrical electrode respectively. This ion current is amplified and fed to a recorder or to the A/D converter of a computer data acquisition system. During the burning process, oxidized or partially oxidized fragments of the solute are created in the flame and these fragments are thought to generate electrons by thermionic emission. The background current (ions and electrons from the hydrogen flame alone) is exceedingly small (1-2 x 10^-12 amperes) and, consequently, the noise level is also commensurably minute (about 10^-14 amperes). The ionization process has a very low efficiency and only 0.0018% of the solute molecules produce ions, (i.e., about two ions or electrons per 10⁵ molecules). Nevertheless, as the noise level is only 10^-14 amperes, the minimum detectable mass of n-heptane is only 2 x 10^-12 g/sec. At a column flow rate of 20 ml/min. this would be equivalent to a minimum detectable concentration of about 3 x 10^-12 g/ml. It is important to emphasize that the detector responds to mass per unit time entering the detector, not mass per unit volume.

As a consequence, the detector response is almost independent of flow rate. It follows, that the FID can be used very easily with capillary columns. Although the very small flow of column eluent is mixed with a much larger flow of hydrogen prior to entering the detector, the diluting effect has no impact on the sensitivity.

The FID will detect virtually all carbon containing solutes, with the exception of a limited number of small molecular compounds such as carbon disulfide, carbon monoxide, etc. In fact, due to its diverse and comprehensive response, it is considered a universal detector.