The flame ionization detector (FID)
is the most sensitive gas chromatographic detector for hydrocarbons
such as butane or hexane. With a linear range for 6 or 7 orders of magnitude
(106 to 107) and limits of detection in the low picogram
or femtogram range, the FID is the most widely and successfully used gas chromatographic detector for volatile
hydrocarbons and many carbon containing compounds.
Molecules that contained only carbon and hydrogen respond best in this detector but the presence of "heteroatoms" in a molecule, such as oxygen, decreases the detector's response. For instance, the FID's methane response (CH4) is fabulous but formaldehyde's (CH2O) is quite poor. Therefore, highly oxygenated molecules or sulfides might best be detected using another detector instead of the FID. Sulfides determination by the flame photometric detector and aldehydes and ketones analyzed with the photoionization detector are alternatives to the use of the FID for those molecules.
As the animations available here show (www.shsu.edu/~chm_tgc/sounds/sound.html) the FID is constructed of a small volume chamber into which the gas chromatograph's capillary column in directly plumbed.
Usually the small diameter capillary is fitted directly into the bottom of the detector's flame jet. The gaseous eluents from the column are mixed with separately plumbed in hydrogen and air and all are burned on the jet's tip. After the fuel (H2) and oxidant (O2 in air) are begun, the flame is lit using a electronic ignitor, actually an electrically heated filament that is turned on only to light the flame.
The charged particles created in
that combustion process create a current between the detector's electrodes.
One electrode is actually the metallic jet itself, another is close by
and above the jet. The gaseous products leave the detector chamber via
the exhaust. The detector housing is heated so that gases produced by the
combustion (mainly water) do not condense in the detector before leaving
the detector chimney.
The total volume of gas in the FID that
yields the most sensitive and widest linear response is not the same volume of gas when the column effluent flow (~ 1 mL/min) and hydrogen
and air flows are flowing; these gases' total flow into the detector is too
small. Another way to say this is that the optimum column flow to maintain the best
chromatography and the best fuel and oxidant flows for the best flame conditions--all
added together--don't create the best gas flow for the FID detector's design.
This means that to maintain the best analytical conditions, additional gas must
be constantly flowed into the detector. This gas makes up the additional
needed gas flow and so is termed makeup gas. Since the makeup gas needs to be
inert so that its addition doesn't upset the fuel and oxidant balance and since
it needs to be added in relatively large amounts (~30+ ml/min in some detector
designs), nitrogen is usually the gas of choice. Helium would work also but is
a nonrenewable resource and more expensive. All gas flows are controlled by
adjustable gas regulators.
Ignition and Shut Down
The process of lighting the FID goes like this: fuel is turned on at a predetermined flow rate (controlled by the H2 tank's pressure regulator); air on; ignitor is lit and the flame ignited. After the flame is confirmed burning, makeup gas flow is turned on. The flame stabilizes within an hour or less and then is routinely left on continuously to maintain the lowest signal background and therefore produce the lowest detection limits. Some labs with high sample throughputs keep FID flames burning continuously, only shutting the fame off when gas tanks need be replaced. GC column carrier is also obviously keep constantly flowing.
Turning off the flame involves first shutting off the fuel flow which extinguishing the flame, then the oxidant and makeup gas flows are closed.
These notes were written by Dr. Thomas G. Chasteen; Department of Chemistry, Sam Houston State University, Huntsville, Texas 77341.
© 2000, 2007, 2009.