Flame ionisation detectors (analysers)

Another type of a gas detector is widely used not only in the gas chromatography, but as a separate gas analyser as well. At present it is the most sensitive gas detector used in gas chromatography. Its operational principle is based on the measurement of ion current which is developed in a flame where the ionisation of organic compounds takes place. This detector is sensitive to organic compounds, whereas it does not give response for inorganic gases.

Figure 7.9 shows a schematic view of a flame ionisation detector. A carrier gas 1 (usually nitrogen) from the outlet of a separating column of a gas chromatograph is mixed with hydrogen 2, and this mixture is burned in the atmosphere of air 3 in the detector jet 4. Since even small amounts of compressor oil could effect the results of measurements, molecular sieves or a diffusion disc 5 are used to eliminate the oil from the supplied air. An ignitor coil 6 is used to ignite the nitrogen-hydrogen-air mixture leaving the jet. When this gas mixture burns very few ions are developed. A collector of ions 7 is placed above the jet close to the flame. In order to maintain ion flow from the jet to the collector the latter is kept at some positive electric potential (75-150 V) above the normally grounded jet. Thus produced the background ion current passes through the conductors via the insulated feed-through 8 to an amplifier. Then the background current is balanced (brought to zero). Now, when an organic compound leaves the separating column of the gas chromatograph, enters and burned in the jet, large amount of ions are formed in the flame. This increases the ion current (up to 10^-12 A), which is amplified in an amplifier. The compound burned in the flame. Thus we can evaluate the amount of the organic component in the analysed gas mixture.value of this current is proportional to the amount of the organic.

Figure 7.9. Schematic of a flame ionisation detector (analyser).

For simple molecules the detector response is proportional to the carbon content in the compound, whereas in the case of other compounds the response is not easy predictable. These detectors are not as temperature sensitive as thermal conductivity detectors. As we mentioned above, these detectors can be used for continuously measurements of the total hydrocarbon content in the gas mixture. In this case they are called hydrocarbon analysers.


Article Source:: Dr. Alexander Badalyan, University of South Australia