Capillary Chromatography - Flame-Ionization-Detector 2
A few hundred volts is applied between the jet and the electrode and, when a solute containing carbon is eluted from the column and burnt with the hydrogen at the jet, the electron/ion pairs that are formed during combustion are collected at the jet and cylindrical electrode. This current is amplified and fed to a recorder or (in modern chromatographs) to the A/D converter of a computer data acquisition system. A simple diagram of the FID is shown in figure 13.
During solute oxidation, oxidized or partially oxidized fragments of the solute are formed in the flame. These fragments are thought to generate electrons by thermionic emission, which, together with any ions, are collected by a potential of 110 or 200 volts applied between the jet and the electrode. The standing current (ions and electrons produced by the hydrogen flame alone) is very small (1Ð2 x 10-12 amperes) and consequently, the noise level is also commensurably minute (about 10-14 amperes).
The ionization process is very inefficient (only 0.0018% of the solute molecules produce ions which is about two ions or electrons per 105 molecules). However, as the noise level is very small, the minimum detectable mass of n-heptane is only 2 x 10-12 g/s. At a column flow rate of 20 ml/min., this is equivalent to a minimum detectable concentration of about 3 x 10-12 g/ml. As already stated the detector responds to mass per unit time entering the detector, not mass per unit volume, consequently, the response is virtually independent of flow rate. It follows that the FID is excellent for use with capillary columns. The FID detects nearly all carbon containing solutes, with the exception of a limited number of small molecular compounds such as carbon disulfide, carbon monoxide, etc. Due to its diverse and comprehensive response, it is considered a universal detector.