Several different detector designs exist for process gas chromatographs. The two most common are the flame ionization detector (FID) and the thermal conductivity detector (TCD). Other detector types include the flame photometric detector (FPD), photoionization detector (PID), nitrogenphosphorus detector (NPD), and electron capture detector (ECD). All chromatograph detectors exploit some physical difference between the solutes and the carrier gas itself which acts as a gaseous solvent, so that the detector may be able to detect the passage of solute molecules (sample gas components) among carrier molecules.
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Flame Ionization Detectors
Flame ionization detectors work on the principle of ions liberated in the combustion of the sample species. Here, the assumption is that sample compounds will ionize inside of a flame, whereas the carrier gas will not. A permanent flame (usually fueled by hydrogen gas which produces negligible ions in combustion) serves to ionize any gas molecules exiting the chromatograph column that are not carrier gas. Common carrier gases used with FID sensors are helium and nitrogen, which also produce negligible ions in a flame. Molecules of sample encountering the flame ionize, causing the flame to become more electrically conductive than it was with only hydrogen and carrier gas. This conductivity causes the detector circuit to respond with a measurable electrical signal.
A simplified diagram of an FID is shown here:
Hydrocarbon molecules happen to easily ionize during combustion, which makes the FID sensor well-suited for GC analysis in the petrochemical industries where hydrocarbon composition is the most common form of analytical measurement. It should be noted, however, that not all carboncontaining compounds significantly ionize in a flame. Examples of non-ionizing organic compounds include carbon monoxide, carbon dioxide, and carbon sulfide. Other gases of common industrial interest such as water, hydrogen sulfide, sulfur dioxide, and ammonia likewise fail to ionize in a flame and thus are undetectable using an FID.
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