Flame ionization Analyzer:

The flame ionization method of hydrocarbon analysis relies on a highly sensitive, but non-selective detector (FID) which decomposes the gas sample in a hydrogen flame. The gas sample is destroyed in the process so no further analysis of the sample is possible. Variations on this type of detector are commonly found in gas chromatographs, on-line hydrocarbon analyzers and in portable field survey instruments. The flame ionization detector alone will provide only total hydrocarbon data however, the detector may be readily enhanced to provide some degree of selectivity in on-line instruments. In GC applications, column seperation is used for speciation.

Here is how the Flame Ionizaton Detector (FID) works:

This is a simple concept and this example uses a simple case. We will follow one methane (CH₄) molecule through the process of ionization in a hydrogen diffusion flame and show how it may then be detected in the hydrocarbon analyzer. A diffusion flame is one in which the oxygen necessary to support combustion is supplied to the outside of the fuel supply. This results in the necessary flame geometry for stripping hydrogen atoms from the hydrocarbon molecule and the formation of carbon radicals.

During the course of normal combustion, a small, stable proportion of CH₄ molecules will temporarily ionize into C⁺ (Carbon, or carbon-containing cation, e.g. CHO+) and e^- components. The flame provides the energy for this ionization. This is generally speaking, a short-lived condition and the charged components quickly recombine into the products of combustion as they loose energy, moving upward, out of the flame. However:

If this happens in an electrostatic field, the oppositely charged components can be driven apart, toward the oppositely charged field generators (in this case, vertical plates, one held at a high negative potential and the other at approximately ground). Upon reaching the oppositely-charged plate, the charged component (C⁺ or e^-) is electrically completed. The Carbon molecule will readily form CO2 with the Oxygen from the surrounding air. Note: the remaining Hydrogen is not a player in this part of the reaction, but readily forms water vapor

Looking more closely at the diagram below, the negative supply is an unremarkable high voltage type which supplies the Carbon cation with electrons, but the "ground" is the input of a sensitive current (I) amplifier which "counts" the electrons (e^-) passing through it on their way to neutralize charge. Indirectly then, for Carbon atoms in an H-C molecule, this scheme acts as a "Carbon Counter". Some variation in response occurs due to species molecule arrangement. This difference (from the "carbon counter" model) is somewhat unique for different species. A " Response Factor" is associated with different hydrocarbon species to correct their measured response with a standard calibration gas.

This is a general explanation applicable to all FIDs found in hydrocarbon analyzers. Different manufacturers use slightly differing schemes and geometries. Some have a single detecting plate, or horizontal ring, some have a grounded flame tip and count Carbon "directly". The detector biasing voltage may vary (between designs) from about -90 to -400 VDC. The FID is usually heated to prevent the formation of water vapor condensation from the FID flame, however a heated FID will have a detector and sample handling components operating at up to 200 degrees C in order to avoid both water vapor and long chain hydrocarbon condensation in the sample gas.

Normal Combustion: i.e. burn methane in air and get carbon dioxide and water vapor...

CH₄ + O₂ --------> CO₂ + H₂O

or:

CH₄ + 3O₂ = CO₂ + 2H₂O

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ion detector operation in a hydrocarbon analyzer