FID VOC Analysers
The FID method is based on the ionization of organically bonded carbon atoms in a hydrogen flame and the discharge of the ions produced to a pair of electrodes. The ionic current is measured. The current is proportional, over a wide range, to the number of carbon atoms delivered to the flame per unit time. The molecular structure (e.g. single or double bond, number and nature of hetero-atoms, chain length and ring- structure) has a considerable influcnce on the oxidation properties of the carbon and thereby on the height of the detector signal. Consequently, organic compounds with oxygen as hetero-atom are generally indicated with far less sensitivity than pure hydrocarbons with the same number of hydrogen atoms per molecule. The different sensitivities to different organic compounds are expressed as response factors. These are specific to substance and apparatus (dependence on nozzle shape, electrode, combustion chamber, and combustion air admixture) and cannot be transferred to other FIDs.
The flame ionization measurement is suitable for the direct proof of organically bound carbon. It makes measurements possible within the trace range of single digit parts per million (ppm) up to process concentrations in percent (%) level. In gas chromatography the flame ionization detector (FID) has great importance. Increasing interest in measurements in environmental protection and process monitoring promoted the developments for one application of FID to the proof of hydrocarbons in the atmosphere. Forfor emission measurements, in particular with unburned hydrocarbons in automobile exhaust gases, substantially larger measuring range is applied. Further areas of application of FID other than total hydrocarbon measurements, are surveillance of plants or areas for toxic concentrations of solvent or the leakage detection and monitoring of fuel stores.