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FID DETECTOR The FID detector is the most commonly used GC detector, responding linearly from its minimum detectable quantity of about 100 picograms to almost 100%. The FID response is very stable from day to day, and is not susceptible to contamination from dirty samples or column bleed. The FID detector responds to any molecule with a carbon-hydrogen bond, but not at all or poorly to compounds such as H2S, CCl4, or NH3. The carrier gas effluent from the GC column is mixed with hydrogen and then routed through an unbreakable stainless steel jet. The hydrogen supports a flame at the tip of the jet, ionizing the analyte molecules. A collector electrode attracts the negative ions to the electrometer amplifier producing an analog signal which is connected to the data system input. Unlike many other FID designs, the SRI FID detector employs a unique ceramic igniter which can run hot continuously, thus totally preventing the flame from extinguishing even when presented with large water injections or pressure surges from column backflush. The FID is thermostatted in an aluminum block up to 375°C and is equipped with a electrometer amplifier which has high, medium, and low gain settings. Hydrogen and air gas flows are controlled using Electronic Pressure Control ( EPC ) for high precision. The optional built-in "whisper quiet" air compressor ( part# 8690-0070 ) is often used to supply the air for the FID, eliminating the bulky air cylinder.
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