Flame ionization detector
Encyclopedia
A flame ionization detector (FID) is a type of gas detector
used in gas chromatography. The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne
, Australia
.
The detection of organic compound
s is most effectively done with flame ionization. Biochemical compounds such as proteins, nucleotides, and pharmaceuticals can be studied with flame ionization as well as other detectors, like thermal conductivity
, thermionic
, or electrolytic conductivity due to the presence of nitrogen, phosphorus, or sulfur atoms or because of the universality of the thermal conductivity detector. However, typically the biochemical compounds have a greater amount of carbon present than other elements. This means that a particular compound may be more easily detected using flame ionization over the other methods because of higher carbon concentration and also flame ionization's sensitivity.
FIDs are best for detecting hydrocarbons and other easily flammable components. They are very sensitive to these components, and response tends to be linear across a wide range of concentrations.
However, an FID destroys most, if not all, of the components it is detecting. Contrarily, with a TCD
the components can continue on to another detector after passing through the TCD; thus it is considered a non-destructive detector (this can be useful for analyzing complex mixtures where different detectors are needed because of differing detector selectivities). However, with an FID, most components are destroyed and no further detection is possible.
For this reason, in multiple-detector situations, the FID is almost always the last detector. An FID essentially can only detect components which can be burned
. Other components may be ionized by simply passing through the FID's flame, but they tend not to create enough signal to rise above the noise of the detector.
FIDs can also be integrated into portable measurement devices and used, for example, for Landfill gas monitoring
and fugitive emissions
monitoring.
most organic compounds, producing positively charged ions and electrons.
In order to detect these ions, two electrodes are used to provide a potential difference. The positive electrode doubles as the nozzle head where the flame is produced. The other, negative electrode is positioned above the flame. 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 ions thus are attracted to the collector plate and upon 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 displayed is based on the computer and software. In general, a graph is displayed that has time on the x-axis and total ion on the y-axis.
The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame. Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms (ions) hitting the detector per unit time. This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate.
The eluent
exits the GC column (A) and enters the FID detector’s oven (B). The oven is needed to make sure that as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on the interface between the column and FID. This deposition would result in loss of effluent and errors in detection. As the eluent travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). The eluent/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. The ions are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal (H) to an amplifier, integrator, and display system. The products of the flame are finally vented out of the detector through the exhaust port (J).
s such as carbonyl, alcohol, halogens, or amines 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 it comes off a gas chromatograph column. Another drawback is the sample is destroyed, making it impossible to use the sample for other measurements. For this reason the FID is typically the final detector or stage in a series of instruments.
Some of the benefits of a flame ionization detector are quite useful. FIDs are insensitive to H2O, CO2, CS2, SO2, CO, NOx, and noble gases because they are not able to be oxidized/ionized by the flame. This allows samples to be studied even if contaminated or if some leakage of ambient room gases occurs at the time of the injection. Additionally, it has the ability to determine when a sample will elute off the column with regards to the solvents used. Some detectors can be damaged if an effluent too concentrated is analyzed, making it necessary to turn it off to prevent damage. The FID is rugged, meaning that column parameters can be tested to find good separation of the constituents of a sample in the column and also from the solvent so delays can be added to more sensitive instruments to prevent damage.
Gas detector
A gas detector is a device which detects the presence of various gases within an area, usually as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down...
used in gas chromatography. The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne
Melbourne
Melbourne is the capital and most populous city in the state of Victoria, and the second most populous city in Australia. The Melbourne City Centre is the hub of the greater metropolitan area and the Census statistical division—of which "Melbourne" is the common name. As of June 2009, the greater...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
.
The detection of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s is most effectively done with flame ionization. Biochemical compounds such as proteins, nucleotides, and pharmaceuticals can be studied with flame ionization as well as other detectors, like thermal conductivity
Thermal conductivity detector
The thermal conductivity detector , also known as a Katharometer, is a bulk property detector and a chemical specific detector commonly used in gas-liquid chromatography. This detector senses changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas...
, thermionic
Nitrogen Phosphorus Detector
The nitrogen phosphorus detector is a form of gas chromatography in which thermal energy is used to ionize an analyte. With this method, nitrogen and phosphorus can be selectively detected with a sensitivity that is 104 times greater than that for carbon....
, or electrolytic conductivity due to the presence of nitrogen, phosphorus, or sulfur atoms or because of the universality of the thermal conductivity detector. However, typically the biochemical compounds have a greater amount of carbon present than other elements. This means that a particular compound may be more easily detected using flame ionization over the other methods because of higher carbon concentration and also flame ionization's sensitivity.
Application
This selectivity can be a problem or an advantage. For example, an FID is excellent for detecting methane in nitrogen, since it would respond to the 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 across a wide range of concentrations.
However, an FID destroys most, if not all, of the components it is detecting. Contrarily, with a TCD
Thermal conductivity detector
The thermal conductivity detector , also known as a Katharometer, is a bulk property detector and a chemical specific detector commonly used in gas-liquid chromatography. This detector senses changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas...
the components can continue on to another detector after passing through the TCD; thus it is considered a non-destructive detector (this can be useful for analyzing complex mixtures where different detectors are needed because of differing detector selectivities). However, with an FID, most components are destroyed and no further detection is possible.
For this reason, in multiple-detector situations, the FID is almost always the last detector. An FID essentially can only detect components which can be burned
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
. Other components may be ionized by simply passing through the FID's flame, but they tend not to create enough signal to rise above the noise of the detector.
FIDs can also be integrated into portable measurement devices and used, for example, for Landfill gas monitoring
Landfill gas monitoring
Landfill gas monitoring is the process by which gases that are released from landfills are electronically monitored.-Techniques for the monitoring of landfill gas:...
and fugitive emissions
Fugitive emissions
Fugitive emissions are emissions of gases or vapors from pressurized equipment due to leaks and various other unintended or irregular releases of gases, mostly from industrial activities. As well as the economic cost of lost commodities, fugitive emissions contribute to air pollution and climate...
monitoring.
Operation
As the name suggests, analysis involves the detection of ions. The source of these ions is a small hydrogen-air flame. Sometimes hydrogen-oxygen flames are used due to an ability to increase detection sensitivity, however for most analysis, the use of compressed breathable air is sufficient. The resulting flame burns at such a temperature as to pyrolyzePyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
most organic compounds, producing positively charged ions and electrons.
In order to detect these ions, two electrodes are used to provide a potential difference. The positive electrode doubles as the nozzle head where the flame is produced. The other, negative electrode is positioned above the flame. 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 ions thus are attracted to the collector plate and upon hitting the plate, induce a current. This current is measured with a high-impedance picoammeter and fed into an integrator
Integrator
An integrator is a device to perform the mathematical operation known as integration, a fundamental operation in calculus.The integration function is often part of engineering, physics, mechanical, chemical and scientific calculations....
. How the final data is displayed is based on the computer and software. In general, a graph is displayed that has time on the x-axis and total ion on the y-axis.
The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame. Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms (ions) hitting the detector per unit time. This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate.
Operating principles
The design of the flame ionization detector varies from manufacturer to manufacturer, but the principles are the same. Most commonly, the FID is attached to a gas chromatography system.The eluent
Elution
Elution is a term used in analytical and organic chemistry to describe the process of extracting one material from another by washing with a solvent ....
exits the GC column (A) and enters the FID detector’s oven (B). The oven is needed to make sure that as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on the interface between the column and FID. This deposition would result in loss of effluent and errors in detection. As the eluent travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). The eluent/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. The ions are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal (H) to an amplifier, integrator, and display system. The products of the flame are finally vented out of the detector through the exhaust port (J).
Additional Considerations
However, one thing to note is that the FID is detecting oxidized carbon atoms in ion form. In organic species that already have oxidized carbons via the presence of oxygen, a weaker signal is given when the sample enters the detector because the oxidized carbons are not ionized as effectively as compared to compounds solely of carbon and hydrogen. Functional groupFunctional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
s such as carbonyl, alcohol, halogens, or amines 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 it comes off a gas chromatograph column. Another drawback is the sample is destroyed, making it impossible to use the sample for other measurements. For this reason the FID is typically the final detector or stage in a series of instruments.
Some of the benefits of a flame ionization detector are quite useful. FIDs are insensitive to H2O, CO2, CS2, SO2, CO, NOx, and noble gases because they are not able to be oxidized/ionized by the flame. This allows samples to be studied even if contaminated or if some leakage of ambient room gases occurs at the time of the injection. Additionally, it has the ability to determine when a sample will elute off the column with regards to the solvents used. Some detectors can be damaged if an effluent too concentrated is analyzed, making it necessary to turn it off to prevent damage. The FID is rugged, meaning that column parameters can be tested to find good separation of the constituents of a sample in the column and also from the solvent so delays can be added to more sensitive instruments to prevent damage.
Sources
- Skoog, Douglas A., F. James Holler, & Stanley R. Crouch. Principles of Instrumental Analysis. 6th Edition. United States: Thomson Brooks/Cole, 2007.
- Halász, I. & W. Schneider. “Quantitative Gas Chromatographic Analysis of Hydrocarbons with Capillary Column and Flame Ionization Detector.” Analytical Chemistry. 33, 8 (July 1961): 978-982