Flame ionization detector

A flame ionization detector (FID) is a type of detector used in gas chromatography.

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Principle

The Flame Ionization Detector (FID) is one of the many methods by which to analyze materials coming off of gas chromatography column. Below is a chart of the typical types of Gas Chromatography Detectors and their uses, taken from Principles of Instrumental Analysis (Skoog, 2007: 793).

The detection of organic compounds 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 ionizations sensitivity.

Application

Ionization essentially can only detect components which can be burned. Other components may be ionized by simply passing through the FIDs flame, but they tend not to create enough signal to rise above the noise of the detector.

This selectivity can be a problem or an advantage. For example, an Ionization is excellent for detecting methane in nitrogen, since it would respond to the methane but not to the nitrogen.

Ionization 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, Ionization destroy most, if not all of the components they are detecting. For example, with a TCD the components can continue on to another detector after passing through the TCD, as 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 Ionization, most components are destroyed and no further detection is possible.

For this reason, in multiple-detector situations, the Ionization is almost always the last detector.

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 such a temperature as to pyrolyze most organic compounds, producing positively charged ions and electrons.

In order to detect these ions, two electrodes are used to provide a potential. 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 integrater. 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 signal collected 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.

Schematic

The design of the Flame Ionization Detector varies from manufacturer but the principles are the same in any case. Most commonly, the FID is attached to a Gas Chromatography system, so for the sake of explanation, that is the starting point that this schematic will take.  

The effluent 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 effluent 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 effluent travels up the FID, it first mixed with the hydrogen fuel (C) and then the oxidant (D). The effluent/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 elluent. 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 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 groups 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 a FID to detect effluent as it comes off a gas chromatograph column. Another drawback is the sample is destroyed, making it nearly impossible to make determinations about the structure or composition. It is 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, 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

For a more detailed schematic, see http://ustl1.univ-lille1.fr/chimie/html/Enseignement/ATE_web/chrom/FID.GIF