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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.