Flame ionization detector

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Structure of an FID: GC column (A), heater (B), hydrogen (C), combustion air (D), power supply (E), flame (F), collector plates (G), signal pick-up (H), exhaust gas (J)

The flame ionization detector - FID for short - is a detector for organic compounds that is mainly used in connection with gas chromatographs (GC). Further areas of use of the FID, the effluent monitoring volatile hydrocarbon-containing substances (with pre-stripping), the monitoring of the methane gas emissions in landfills as well as the space and outside air monitoring for hydrocarbons.

Working principle

The functional principle is the measurement of the conductivity of an oxyhydrogen flame (the fuel gas is hydrogen) between two electrodes . To be analyzed substances are transported by a carrier gas stream into the flame and there thermally ionized . The electrons released during ionization are captured (e.g. by means of a grid or curved capacitor plates that are attached to the flame) and recorded as a peak by a connected recorder or a data system . The detectable current is in the picoampere range, so that it has to be amplified accordingly by precise electronics for monitoring.

The FID is the most widely used detector in gas chromatography , as it combines robustness with high sensitivity. An FID is up to 10 3 times more sensitive than a thermal conductivity detector . In addition, the detector signal is linearly proportional to the amount of analyte (more precisely, to its carbon content) over a wide concentration range. Therefore, the concentration of a hydrocarbon can be estimated from the signal without calibration , so that the detector can be used well for quantification.

However, some organic substances (e.g. formic acid, acetaldehyde) are more difficult to detect because they are thermally decomposed beforehand in the column (e.g. formic acid, which breaks down into carbon monoxide and water ). Substances that respond little or no response are:

Noble gases, H 2 , N 2 , nitrogen oxides, CO , CCl 4 or other halogenated compounds, silicon halides, CO 2 , H 2 O , CS 2 , NH 3 , O 2
detector WLD FID NPD ECD MS
Detection limit 1 µg 1 ng 10 pg 1 pg up to 1 fg

The detection limit depends not only on the dosage but also on the substance to be analyzed.

In contrast to the ECD or WLD, the FID has a destructive effect. This means that the sample to be analyzed (here by burning) is destroyed.

Related devices

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