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TRACES Centre GC Detector Series Flame Ionization Detector: FID A carrier gas (usually He or H2) from the column enters at the bottom of the detector and is mixed with hydrogen combustion gas plus optional make-up gas in the area below the flame jet. This mixture is then combined with air and burned just above the jet tip. A negative polarizing voltage is applied between the jet tip and a collector electrode; as electrons are formed, they are accelerated across the jet tip–collector gap by the electric field and sent to an electrometer. Depending upon the FID design, either the collector or the jet tip is kept at ground potential. Air, carbon dioxide and water exhaust gases are vented from the top of the detector body. In some flame ionization detectors, a glow-plug operates momentarily to ignite the flame. Usually a 200 V polarization voltage is applied across the flame jet and the collector. Electrons formed in the flame by combustion of hydrocarbons are collected under the influence of the electrical field, and the resulting current is converted to a voltage by an electrometer that can have one or more operating ranges. The voltage is amplified and high-frequency components are filtered out. The detector signal is converted to discrete digital samples by an A/D converter and additional signal processing is applied as required.