The Flame Ionization Detector

The FID was developed in 1958 by McWilliam and Dewar in Australia and almost simultaneously by Harley, Nell and Pretorius in South Africa and quickly became the detector of choice in commercial instrumentation. As an ionization detector, the FID responds readily to compounds that contain carbon and hydrogen and to a lesser extent to some compounds containing only carbon. It is unresponsive to water, air and most carrier gases. Because of its broad applicability and relative ease of operation, it is probably the most common detector in GC systems. The FID responds quickly and can be constructed with a small internal volume, which makes it especially well suited for capillary GC.

Response of an FID is due to the sample being burned in a fuel-rich mixture and producing ions. In the same process, electrons are produced. Either ions or electrons are collected at an electrode and produce a small current. Since there are virtually no ions present in the absence of sample, the baseline is stable and the current is easily converted to a voltage and amplified to produce a signal. The response to most hydrocarbons is about 0.015 C g_1 carbon.

As shown in Figure 1, the most often used configuration has the jet tip at approximately 200 V relative to the collecting electrode. For use with capillary columns, a smaller jet tip (c. 0.3 mm i.d. rather than the 0.5 mm used with packed column configurations) is utilized in order to increase detector sensitivity. The capillary column is usually inserted through the ferrule and then a few centimetres are broken off and discarded. Ideally, the column is positioned within 1-2 mm of the jet tip and column effluent enters the detector and mixes with hydrogen (fuel) and make-up gas without undue contact with metal surfaces. This

Collector

Jet tip Insulator Jet base

Air in

Carrier gas in [from column!

Figure 1 Cross-sectional diagram of a flame ionization detector.

mixture is combusted in an excess of air and the organic components are decomposed into ions. The ion chemistry of the diffusion flame has been studied by mass spectrometry. It appears that the ultimate positive charge carrier is H3O + (or clusters of this with water molecules) resulting from charge transfer reactions from the initially formed ions (principally CHO + ). Thus the detector is often referred to as providing an 'equal per carbon response'.

This response to hydrocarbons allows one to quantify mixtures, for example from petroleum samples, without necessarily identifying each of the components individually present. With compounds other than hydrocarbons, the response is decreased when partially oxidized carbon atoms are present. This requires corrections to be made when the compounds contain, for example, oxygen, nitrogen or halogens. Either pure sample compounds or compounds of similar structure are used to establish appropriate response factors. Alternatively, the concept of effective carbon number has been updated to provide a model for the quantification of components in a complex organic mixture if they can be assigned to general functional group categories.

When used with narrow capillary columns, the FID usually requires a make-up gas for maximum sensitivity. The wider (530 |im) columns can be operated at

To electrometer

To electrometer

Collector

Jet tip Insulator Jet base

Air in

Carrier gas in [from column!

Figure 1 Cross-sectional diagram of a flame ionization detector.

a higher carrier gas flow rate and may often be used without the additional make-up gas. For most operations the total flow rate (column + make-up) will be 20-60 mL min"1. The fuel and air flow rates are maintained close to that recommended by the manufacturer - often 30-40 mLmin"1 for the hydrogen fuel and about 10-fold higher for the air. Under these conditions, the minimum detectable amount (MDA) of organic compounds is approximately 10-100 pg, depending on the structure. In addition, response is usually linear from the MDA to a concentration some 107 times as great. (This higher limit is often beyond the loading capacity of narrow-bore capillary columns.) Flows to the detector can be adjusted while using standard samples containing the components of interest in order to obtain maximum response.

Water is a product of the combustion process producing the ions. Thus, the detector assembly must be kept hot in order to prevent condensation. A convenient guide is to have the detector 20-50° greater than the upper column temperature, but in no case lower than 150°C. Then water vapour, along with the other combustion gases, is swept out of the detector body. With most instruments, once the thermal environment of the detector has stabilized, temperature fluctuations are small and easily tolerated.

The FID is often described as a 'forgiving' detector since acceptable results are obtained even when the gas flows and other conditions are not optimized. None the less, some caution must be taken to avoid baseline drift, loss of sensitivity and the presence of spurious peaks. It is important to ensure that the gases employed are free from hydrocarbon impurities. Filters are available for this purpose. The flame itself is quite small and invisible, so checking for the presence of water vapour is the best approach to ensure that flame ignition has been successful. This can be done by holding a cold mirror above the outlet of the detector and observing condensation of the water vapour. Deterioration of performance of a properly operating FID is often the result of having used chlorinated solvents. Soot particles and the presence of HCl eventually lead to high and noisy baselines. The jet tip and collector electrode may have to be cleaned or, if badly corroded, replaced. Some spiking may be observed if portions of the polyimide coating are burned off the end of the capillary column.

The FID is mass flow-sensitive, meaning that the area response for a compound does not change as flow rate is varied. For quantitative work, appropriate response factors must be obtained, especially if a split injection mode is employed. When properly configured, a FID can respond to approximately 20 pg of each component eluting from a high resolution capillary column.

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