Flame Ionization Detector (FID)

Pure hydrogen-air flames are practically ion-free but on introduction of even little amount of hydrocarbons the flame causes considerable ionization and becomes electrically conducting. The ionization current is proportional to the number of carbon atoms present in the hydrocarbon molecules. Thus, FID is effectively a carbon atom counter e.g., one molecule of propane generates three times the response generated by one molecule of methane. The measurement of HC by FID is expressed as parts per million of methane i.e. as ppmC ₁ i.e., ppm of hydrocarbon containing equivalent of one carbon atom. The HC concentration is commonly written as ppmC. HC concentration measured as ppm propane (C₃ ) is to be multiplied by a factor of 3 to convert it to ppmC. All classes of hydrocarbons i.e., paraffin, olefins, aromatics, etc. show practically the same response to FID. Oxygenates, e.g. aldehydes and alcohols however, have a somewhat lower response.

FID essentially consists of a hydrogen-air burner and an ion collector assembly as shown in Fig. 4.11. Sample gas is introduced with hydrogen in the burner assembly and the mixture is burned in a diffusion flame. An electric potential is applied between the collector plates that makes the ionization current to flow and generate signal proportional to HC concentration in the sample gas. This current is amplified and the output signal is measured.

A well-designed burner will generate ionization current that is linearly proportion to hydrocarbon content over a dynamic range of almost 1 to 10 ⁶ . The commercial FID analyzers have the most sensitive range set at about 0-50 ppmC and the maximum range reaching 0-100,000 ppmC.

Hydrogen is mixed with helium in ratio of 40:60 to decrease flame temperature that increases flame stability. The FID analyzer is calibrated with propane or methane mixtures in nitrogen. For the measurement of hydrocarbons in diesel exhaust, sampling line and FID are heated to a temperature of 191± 11°C to minimize condensation of heavy hydrocarbons present in the diesel exhaust in the sampling system.