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Flame Ionization: GC's Workhorse Detector | LCGC
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Flame Ionization: GC's Workhorse
Detector
Jul 01, 2015 By John V. Hinshaw
LCGC North America
Volume 33, Issue 7, pg 470–477
Nuts and Bolts
A fraction of the carbon-based compounds that pass through the FID flame
are ionized, something like one in 10,000 (3). In most modern designs, the
electrons formed inside the detector are impelled by an electric potential
toward a collector electrode, producing a minute current on the order of
picoamperes (10-12 A). This current is converted to a voltage, filtered, and
amplified as required. Even though not all of the carbon molecules are
ionized, FID can deliver a limit of detection on the order of a few picograms of
carbon per second.
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The internal arrangement of a typical flame ionization detector is shown in
Figure 1.
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Flame Ionization: GC's Workhorse Detector | LCGC
Figure 1: Flame ionization detector cross-section. 1 = electrometer connection, 2 =
effluent exit, 3 = igniter coil, 4 = igniter power connection, 5 = polarizing voltage supply
connection, 6 = air input, 7 = column connection, 8 = hydrogen input, 9 = flame jet, 10
= collector electrode. (Adapted from a figure courtesy of PerkinElmer Instruments.)
Carrier gas from the column enters at the bottom of the detector and is mixed
with hydrogen combustion gas plus optional makeup gas in the area below
the flame jet. This mixture is then combined with an excess of air and burned
just above the jet tip. A negative polarizing potential 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. Depending on the
detector design, either the collector or the jet tip is kept at ground potential;
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Flame Ionization: GC's Workhorse Detector | LCGC
Figure 1 shows a grounded collector design with the electrometer input at
virtual ground and the jet tip charged to -200 V. Air, carbon dioxide, and
water exhaust gases are vented from the top of the detector body. In most
modern FID systems a glow plug operates momentarily to ignite the flame.
The electric current from the collector is converted to a voltage by an
electrometer and then is amplified, filtered, digitized, and processed as
required. Signal zeroing, scaling, and attenuation are performed in the
instrument firmware. The detector signal is filtered to remove unwanted highfrequency noise. Noise is produced by instabilities in the flux of ionizable
impurities in the carrier gas, by the flame itself, by the electronic circuit, and
by induction of stray electromagnetic signals from, for example, nearby cell
phones. Excessive noise usually is the result of contamination from a buildup
of carbon particles in the detector, poor electrical connections, or incorrect
operational settings.
Sensitivity
FID sensitivity depends on the combustion gas flow rates, the carrier gas flow
rate, the flame jet exit diameter, the electric field formed by the relative
positions and shape of jet and collector, and to a lesser degree the detector
temperature.
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