A flame ionisation detector (FID) is one of the most common measurement principles for hydrocarbons. It can either be used alone, or combined with gas chromatography, GC-FID.
To create the flame in an FID detector, hydrogen and air are burned together to form a high temperature flame through which the analyte gas sample flows. For vehicle emissions testing applications, it is common to use a mixture of 40% Hydrogen in Helium as the fuel gas. In the petrochemical sector, use of pure hydrogen is prevalent. Dilution of the flame with helium (which is an excellent heat conductor) reduces the flame temperature and allows a more sensitive analysis.
Some FID instrumentation is customised to have an extremely rapid response time and can be used to measure changes in the total concentration of unburned hydrocarbons in an internal combustion engine exhaust gas many hundreds of times during a single piston stroke. This rapid response characteristic also makes FID instrumentation suitable for use in chemical process control loops, where a rapid flow of information about process conditions is essential to enable fine tuning of the relevant process parameters.
As the various chemicals in the sample are burned (ionised) in the flame, their ionisation energy is measured and this can be used to establish their concentration in the gas stream. To achieve speciation within a mixed hydrocarbon sample, the FID should be combined with a chromatography technique such as gas chromatography (GC), thin layer chromatography (TLC) or high pressure liquid chromatography (HPLC).
A GC-FID setup is common in hydrocarbon processing process control measurement applications and analysis of VOCs in air. A range of specialty gases calibration mixtures, containing similar species to the sample and at similar concentrations, will be required to calibrate the GC-FID setup. The gas chromatography carrier gas, such as pure helium, hydrogen or nitrogen, doubles up as a zero gas for the instrument.
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