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Considered as almost universal for organic compounds, this is the detector par excellence for GC. The gas flow issuing from the column passes through the flame of a small burner fed by a mixture of hydrogen and air. When a compound is eluted, its combustion results in the release of ions and charged particles responsible for the passage of a very weak current (10⁻¹² μA) between two electrodes (p.d. of 100–300 V). The signal is transformed by an electrometer into a measurable voltage (Figure 2.12). One end of the burner, held at ground potential, acts as a polarization (ground) electrode, while the ring‐shaped second electrode surrounds the flame.

Figure 2.12 FID detector (a) and NPD detector (b). The make‐up gas (or filling gas) is useful when the flow rate of the capillary column is too small. The electrometers used with detectors enable the measurement of intensities that would too small for a galvanometer. Many variants exist depending on the manufacturer.

(Source (a): Modified from Cremer, E. and Prior, F. (1951), Anwendung der chromatographischen Methode zur Trennung von Gasen und zur Bestimmung von Adsorptionsenergien. Zeitschrift für Elektrochemie und angewandte physikalische Chemie, 55, 66–70. https://doi.org/10.1002/bbpc.19510550115. (b): Courtesy of Supelco.)

For organic compounds, the intensity of the signal is sensitive to the mass flow of the sample, except in the presence of heteroelements, such as halogens. The latter may change the response and several simple compounds, such as water, carbon dioxide or ammonia, do not give any response. Thus, the area under the peak reflects the mass m of the compound eluted (dm/dt integrated between the beginning and end of the peak). An FID detector is not affected by variations in flow rate, which can lead to errors with some types of detectors. The sensitivity of this detector is expressed in Coulombs/g of carbon, and the dead volume of the detector is null. The detection limit is in the order of 2 or 3 pg/s, and the linear dynamic range reaches 10⁸; however, concentrated solutions do not lead to the best resolution.

To evaluate the overall quantity of volatile organic compounds (VOCs) in polluted air, there exist small portable instruments housing a flame ionization detector that allows the measurement of the carbon factor of the atmosphere examined, without prior chromatographic separation.

Figure 2.13 Thermal conductivity detector. Left, layout demonstrating the dual circulation of the carrier gas. Right, a katharometer unit with the principle of its electrical connections in a Wheatstone bridge type assembly.