flame ionization detector - wikipedia, the free encyclopedia

Schematic of a flame ionizationdetector for gas chromatography

Flame ionization detectorFrom Wikipedia, the free encyclopedia

A flame ionization detector (FID) is a scientific instrument that measuresthe concentration of organic species in a gas stream. It is frequently used asa detector in gas chromatography. Standalone FIDs can also be used inapplications such as landfill gas monitoring and fugitive emissionsmonitoring in stationary or portable instruments.

Contents

1 History2 Operating principle3 Advantages and disadvantages

3.1 Advantages3.2 Disadvantages

4 Operation5 Description of a generic detector6 See also7 References8 Sources

History

The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne,Australia.[1][2][3]

Operating principle

The operation of the FID is based on the detection of ions formed during combustion of organic compounds in ahydrogen flame. The generation of these ions is proportional to the concentration of organic species in thesample gas stream. Hydrocarbons generally have molar response factors that are equal to number of carbonatoms in their molecule, while oxygenates and other species that contain heteroatoms tend to have a lowerresponse factor. Carbon monoxide and carbon dioxide are not detectable by FID.

Advantages and disadvantages

Advantages

Flame ionization detectors are used very widely in gas chromatography because of a number of advantages.

Cost: Flame ionization detectors are relatively inexpensive to acquire and operate.Low maintenance requirements: Apart from cleaning or replacing the FID jet, these detectors require no

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maintenance.Rugged construction: FIDs are relatively resistant to misuse.Linearity and detection ranges: FIDs can measure organic substance concentration at very low and veryhigh levels, having a linear response of 10^6.

Disadvantages

Flame ionization detectors cannot differentiate between different organic substances. They also cannot detectinorganic substances. In some systems, CO and CO2 can be detected in the FID using a methanizer, which is abed of Ni catalyst that reduces CO and CO2 to methane, which can be in turn detected by the FID.

Another important disadvantage is that the FID flame oxidizes all compounds that pass through it; allhydrocarbons and oxygenates are oxidized to carbon dioxide and water and other heteroatoms are oxidizedaccording to thermodynamics. For this reason, FIDs tend to be the last in a detector train and also cannot beused for preparatory work.

Operation

In order to detect these ions, two electrodes are used to provide a potential difference. The positive electrodedoubles as the nozzle head where the flame is produced. The other, negative electrode is positioned above theflame. When first designed, the negative electrode was either tear-drop shaped or angular piece of platinum.Today, the design has been modified into a tubular electrode, commonly referred to as a collector plate. The ionsthus are attracted to the collector plate and upon hitting the plate, induce a current. This current is measuredwith a high-impedance picoammeter and fed into an integrator. The manner in which the final data is displayedis based on the computer and software. In general, a graph is displayed that has time on the x-axis and total ionon the y-axis.

The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame. Specificallyhow the ions are produced is not necessarily understood, but the response of the detector is determined by thenumber of carbon atoms (ions) hitting the detector per unit time. This makes the detector sensitive to the massrather than the concentration, which is useful because the response of the detector is not greatly affected bychanges in the carrier gas flow rate.

Description of a generic detector

The design of the flame ionization detector varies from manufacturer to manufacturer, but the principles are thesame. Most commonly, the FID is attached to a gas chromatography system.

The eluent exits the GC column (A) and enters the FID detector’s oven (B). The oven is needed to make surethat as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on theinterface between the column and FID. This deposition would result in loss of eluent and errors in detection. Asthe eluent travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). Theeluent/fuel/oxidant mixture continues to travel up to the nozzle head where a positive bias voltage exists (E).This positive bias helps to repel the reduced carbon ions created by the flame (F) pyrolyzing the eluent. Theions are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, whichdetects the ions hitting the plates, then feeds that signal (H) to an amplifier, integrator, and display system. Theproducts of the flame are finally vented out of the detector through the exhaust port (J).


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FID Schematic

See also

Flame detectorThermal Conductivity DetectorGas ChromatographyActive fire protectionPhotoionization detector

References^ Scott, R. P. W., 1957, Vapour Phase Chromatography, Ed. D. H. Desty(London: Butterworths), p. 131.

1.

^ McWilliam, I. G.; Dewar, R. A. "Flame Ionization Detector for GasChromatography". Nature 181 (4611): 760. Bibcode:1958Natur.181..760M(http://adsabs.harvard.edu/abs/1958Natur.181..760M). doi:10.1038/181760a0(http://dx.doi.org/10.1038%2F181760a0).

2.

^ Morgan, D J (1961). "Construction and operation of a simple flame-ionization detector for gas chromatography" (http://iopscience.iop.org/0950-7671/38/12/321). J. Sci. Instrum. 38 (12): 501.Bibcode:1961JScI...38..501M (http://adsabs.harvard.edu/abs/1961JScI...38..501M). doi:10.1088/0950-7671/38/12/321(http://dx.doi.org/10.1088%2F0950-7671%2F38%2F12%2F321). Retrieved2009-03-18.

3.

SourcesSkoog, Douglas A., F. James Holler, & Stanley R. Crouch. Principles ofInstrumental Analysis. 6th Edition. United States: Thomson Brooks/Cole,2007.Halász, I. & W. Schneider. “Quantitative Gas Chromatographic Analysis ofHydrocarbons with Capillary Column and Flame Ionization Detector.”Analytical Chemistry. 33, 8 (July 1961): 978-982G.H. JEFFERY , J.BASSET , J.MENDHAM , R.C.DENNEY, "VOGEL'STEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS."

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flame ionization detector - wikipedia, the free encyclopedia

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