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' With the function switch in the proper range, the span pot is adjusted until an exact reading is obtained for the calibration gas This span setting is the reference point from which all other readings will be determined ' 4.2 FLAME IONIZATION DETECTOR WITH PORTABLE GAS CHROMATOGRAPH ' In the survey mode, the Flame ionization detector detects the total concentration of many organic gases and vapors In the gas chromatograph (GC) mode, it identifies and measures specific ' compounds In the survey mode, all the organic compounds are ionized and detected at the same time In the GC mode, volatile species are separated in a packed or capillary column 4.2.1 Theory ' The FID uses ionization as the detection method, in the same manner as the HNU, except that ' the ionization is caused by a hydrogen flame, rather than by a UV light The flame has sufficient energy to ionize any organic species with an IP of 15 4 or less The FID, unlike the PID, will detect methane gas Inside the detector chamber, the sample is exposed to a hydrogen flame that ionizes the ' organic vapors When most organic vapors burn, positively charged carbon-containing ions are produced that are collected by a negatively charged collecting electrode in the chamber An electric ' field exists between the conductors surrounding the flame and the collecting electrode As the positive ions are collected, a current proportional to the hydrocarbon concentration is generated on the ' input electrode This current is measured with a preamplifier that has an output signal proportional to the ionization current A signal conducting amplifier is used to amplify the signal from the ' preamplifier and to condition it for subsequent meter or external recorder display An example of an instrument using a FID is the Foxboro OVA, described below 4.2.2 Foxboro Organic Vapor Analyzer ' The Foxboro Organic Vapor Analyzer consists of two major parts 0 - 12 1