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Flame ionization detectorA flame ionization detector (FID) is a type of gas detector used in gaschromatography. The first flame ionization detector was developed in 1957 byscientists working for the cslRo in Melbourne, AustraliaThe detection of organic compounds is most effectively done with flameionization. Biochemical compounds such as proteins, nucleotides, andpharmaceuticals can be studied with flam e ionization as well as other detectors,like thermal conductivity, thermionic, or electrolytic conductivity due to thepresence of nitrogen, phosphorus, or sulfur atoms or because of the uni.izersality ofthe thermal conductivity detector.ApplicationAn FID is excellent for detecting methane in nitrogen, since it would respond tothe methane but not to the nitrogen.FIDs are best for detecting hydrocarbons and other easily flammable components.They are very sensitive to these components, and response tends to be linear acrossa wide range of concentrations.However, an FID destroys most - if not all - of thecomponents it is detecting. Contrarily, with a TCD the components can continueon to another detector after passing through the TCD; thus ii is considered a non-destructive detector (this can be useful for analyzing complex mixtures wheredifferent detectors are needed because of differing detector selectivities)l However,with an FID, most components are destroyed and no further detection ispossible.For this reason, in multiple-detector situations, the FID is almost alwaysthe last detector. An FID essentially can only detect components which can bebumed. Other components may,be ionized by simply passing through the FID'sflame, but they tend not to create enough signal to rise above the noise of thedetector.FlDs can also be integrated into portable measurement devices and used,for example, for Landfill gas monitoring and fugitive emissions monitoring.

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OperationAs the name suggests, analysis involves the detection of ions. The source of theseions is a small hydrogen-air flame. Sometimes hydrogen-oxygen flames are useddue to an ability to increase detection sensitivity, however for most analysis, theuse of compressed breathable air is sufficient. The resulting flame burns at such atemperature as to pyrolyze most organic compounds, producing positively chargedions and electrons.In order to detect these ions, two electrodes are used to provide a potentialdifference. The positive electrode doubles as the nozzle head where the flame isproduced. The other, negative electrode is positioned above.the flame. When firstdesigned, the negative electrode was either tear-drop shaped or angular piece ofplatinum. Today, the design has been modified into a tubular electrode, commonlyreferred to as a collector plate. The ions thus are attracted,to the collector plate andupon hitting the plate, induce a current. This current is measured with a high-impedance picoammeter and fed into an integrator. How the final data is displayedis based on the computer and software. In general, a graph is displayed that hastime on the x-axis and total ion on the y-axis.The curuent measured corresponds roughly to the proportion of reduced carbonatoms in the flame. Specifically how the ions are produced is not necessarilyunderstood, but the response of the detector is determined by the number of carbonatoms (ions) hitting the detector per unit time. This makes the detector sensitive tothe mass rather than the concentration, which is useful because the response of thedetector is not greatly affected by changes in the carrier gas flow rate.

Operating principlesIn the FID, the carrier gas effluent from the GC column is mixed with hydrogen,then routed through an unbreakable stainless steel jet. The hydrogen mix supports adiffusion flame at the jet's tip which ionizes the analyte molecules. Positive andnegative ions are produced as each sample component is eluted into the flame. A

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collector electrode attracts the negative ions to the electrometer amplifier,producing an analog signal for the data system input. An electrostatic field isgenerated by the difference in potential between the positively charged collectorelectrode and the grounded FID jet. Because of the electrostatic field, the negativeions have to flow in the direction of the collector electrode.Theory of OperationThe ratio of air to hydrogen in the combustion mixture should be approximatelyl0:1. If the carrier flow is higher than normal, the combustion ratio may need to beadjusted. Flow is user adjusted through the Electronic Pressure Controllers (EpC);the rates used to generate test chromatograms at the factory are printed on the rightside of the GC in the flow rate chart. The FID temperature must be hot, enough:sothat condensation doesn't occur anywhere in the system; 150oC is sufficient forvolatile analytes; for semi-volatiles, use a higher temperature. In addition to using'the ignitor to light the flame , it may be left on at an intermediate voltage levelto prevent flameout (-750 or 7.5 volts). The igniior is very durable and will last along time, even at high temperatures.The FID hydrogen diffusion flame

lnner flame / pyrolysis zoneH, andcarrier --)effluent Burner jet

Electrostatic fieldThe ratio of air to hydrogen in the combustion mixture should be approximately10:1. If the carrier flow is higher than normal, the combustion ratio may need to beadjusted. Flow is user adjusted through the Electronic Pressure Controil.r, (EpC);the rates used to generate test chromatograms at the factory are printed on the rightside of the GC in the flow rate chart. The FID temperature must be hot .rorgh ,othat condensation doesn't occur anywhere in the system; 150oC is sufficie.rt fo.volatile analytes; for semi-volatiles, use a higher temperature. In addition to usingthe ignitor to light the flame, it may be left on at an intermediate voltage level to

Outer flame / oxidation zoneOxidizing atmosphere

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prevent flameout (-750 or 7.5 volts). The ignitor is very durable and will last a longtime, even at hightemperatures.

Hydrogen gas Flameport assembly

Analytical column

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t-11 ffiffi::lIgnitor

flCompressed airfeeds hydrogen flame

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: Positively chargedj collector electrode /exhaust vent

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Additional ConsiderationsHowever, one thing to note is that the FID is detecting oxidized carbon atoms inion form.In organic species that already have oxidized carbons via the presence ofoxygen, a weaker signal is given when the sample enters the detector because theoxidized carbons are not ionized as effectively as compared to compounds solelyof carbon and hydrogen. Functional groups such as carbonyl, alcohol, halogens, oramines are sources of these oxidized carbons, sometimes causing few if any ions.This points out one of the main drawbacks of using an FID to detect effluent as itcomes off a gas chromatograph column. Another drawback is the sample isdestroyed, making it impossible to use the sample for other measurements. For thisreason the FID is typically the final detector or stage in a series of instruments.Some of the benefitq of a flame ionization detector are quite useful. FIDs areinsensitive to H2O, COz, CS2, SO2, CO, NO*, and noble gases because they are notable to be oxidized/ionized by the flame. This allows samples to be studied even ifcontaminated or if some leakage of ambient room gases occurs at the time of theinjection. Additionally, it has the ability to determine when a sample will elute offthe column with regards to the solvents used. Some detectors can be damaged if aneffluent too concentrated is analyzed, making it necessary to turn it off to preventdamage. The FID is rugged, meaning that column parameters can be tested to findgood separation of the constituents of a sample in the column and also from thesolvent so delays can be added to more sensitive instruments to prevent damage.