Thermo Scientific TVA-1000B Instruction Manual - Geotech ...

Theory of Operation
Flame Ionization Detection (FID)
A Flame Ionization Detector (FID) measures organic compounds by utilizing a flame produced
by the combustion of hydrogen and air. When hydrocarbons in the sample are introduced
to the detection zone, ions are produced by the following reaction:
RH + O → RHO + + e – → H 2 O + CO 2
where
R = carbon compound
A collector electrode with a polarizing voltage is also located within the detector chamber, and
the ions produced by this reaction are attracted to it. As the ions migrate towards the collector,
a current is produced which is directly proportional to the concentration of hydrocarbons
introduced to the flame. This current is then amplified and sent to a microprocessor and/or
analog readout device.
The FID has a wide dynamic range. The effective dynamic range can be further expanded by
use of a dilutor kit which reduces very high volatile organic compounds (VOC) concentrations
to within the dynamic range (or even linear range) of the analyzer. The dilutor kit can
also be used to enrich oxygen deficient samples by adding ambient air that is rich in oxygen
(20.9% usually). Low oxygen can affect the characteristics of the hydrogen flame, causing
readings to be artificially elevated and possibly extinguishing the flame. As a general rule of
thumb, greater than 16% oxygen is required to support the flame. If underground gases or
samples in gas bags are to be measured by an FID, it is advised that the dilutor be used to
combat the problem.
EXHAUST
METER
COLLECTOR
ELECTRODE
IGNITION
FILAMENT
JET
Figure 2. Typical Flame Ionization Detector
AIR SAMPLE
FUEL (H 2 )
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