The Flame Ionization Detector FID

Century organic vapour analysers are factory calibrated to measure total organic vapours according to a standard (methane). Since different organic vapours interact with the flame ionization detector (FID) to varying extents, it is vital that the instrument user be aware of the magnitude of the variation in order to obtain the most accurate data. Each user must determine relative responses for the individual instrument. 

For measuring the inert species, some of which are present in the majority of gases, the thermal-conductivity detector (TCD) is often the detector of choice for gas analyses. Since the TCD is a concentration detector and its sensitivity is lower than that of mass-flow detectors such as the flame-ionization detector (FID), relatively high concentrations of compounds in the carrier gas are needed. This means that packed columns, with their high loadability, are still quite popular for such analyses. 

GC is coupled with many detectors for the analysis of pesticides in wastewater. At the present time the most popular is GC-MS, which will be discussed in more detail later in this section. The flame ionization detector (FID) is another nonselective detector that identifies compounds containing carbon but does not give specific information on chemical structure (but is often used for quantification because of the linear response and sensitivity). Other detectors are specific and only detect certain species or groups of pesticides. They include electron capture,nitrogen-phosphorus, thermionic specific, and flame photometric detectors. The electron capture detector (ECD) is very sensitive to chlorinated organic pesticides, such as the organochlorine compounds (OCs, DDT, dieldrin, etc.). It has a long history of use in many environmental methods,... 

Detectors range from the universal, but less sensitive, to the very sensitive but limited to a particular class of compounds. The thermal conductivity detector (TCD) is the least sensitive but responds to all classes of compounds. Another common detector is the flame ionization detector (FID), which is very sensitive but can only detect organic compounds. Another common and very sensitive detector is called electron capture. This detector is particularly sensitive to halogenated compounds, which can be particularly important when analyzing pollutants such as dichlorodiphenyltrichloroethane (DDT) and polychlorobiphenyl (PCB) compounds. Chapter 13 provides more specific information about chromatographic methods applied to soil analysis. 

No other detector till date has surpassed the flame ionization detector (FID) as a universal gas chromatographic detector. It hardly meets, all the characteristic features of TCD in terms of simplicity, stability, and versatility besides having two distinctly positive plus points ... 

Note However, no universal detector has so far been discovered for HPLC to cater for a wide-spectrum of components as the Flame-Ionization-Detector (FID) used for GC in Chapter 29. 

As a minimum, the flame ionization detector (FID) or the photo-ionization detector (PID) must be available at industrial sites handling hazardous substances. 

The flame ionization detector (FID) can be used for the detection and quantitative estimation of components separated by the GC. Identification of major species can be achieved by a mass spectrometer which can not be used for quantitative analysis of complex mixtures such as coal liquids. 

With enantiomer analysis, however, a linear detector response is indispensible. Thus, for the correct determination of. say, 0.1 % of an enantiomeric impurity, linearity within a concentration range of at least three orders of magnitude is required. It is generally accepted that the flame ionization detector (FID) does fulfill this requirement, but it is recommended that the linear detector response is verified via dilution experiments31. In contrast, the linear response range of the electron capture detector is low, being only two to three orders of magnitude. 

The flame ionization detector (FID) has become the most commonly used detector in GC. This is because it possesses several outstanding features ... 

Thin-layer chromatography (TLC) on silica gel is well known for its separation power for lipids and related compounds. The flame ionization detector (FID) is a universal analytical instrument that offers high sensitivity and linearity for carbon-containing organic compounds. The combination of TLC and FID led to the wide use of the Iatroscan TLC-FID for the analysis of lipid classes. The adoption of the Iatroscan TLC-FID in both academia and industry has generated sufficient data to indicate that TLC-FID is currently one of the most efficient tools for the quantitation of lipids classes (Ackman et al., 1990 Hammond, 1993). 

Pure fluids. Carbon dioxide is often the mobile phase of choice for SFC, since it has relatively mild critical parameters, is nontoxic and inexpensive, chemically inert, and is compatible with a wide variety of detectors including the flame ionization detector (FID) used widely in GC and the UV absorbance detector employed frequently in HPLC (7). The usefulness of carbon dioxide as a mobile phase in many instances is somewhat limited, however, because of its nonpolarity (8), and many polar compounds appear to be insoluble in it. For a sample containing polar compounds, pure carbon dioxide may not be the proper mobile phase. The elution of polar compounds is often difficult and the peak shapes for these polar compounds are sometimes poor. This latter difficulty is commonly observed with nonpolar supercritical fluids and may be due to active sites on the stationary phase rather than any inherent deficiency in the fluid itself. 

Experimental (simplex and window diagram). The chromatographic system consisted of a Model 501 supercritical fluid chromatograph (Lee Scientific, Salt Lake City, Utah) with the flame ionization detector (FID) set at 375°C. The instrument was controlled with a Zenith AT computer. A pneumatically driven injector with a 200 nL or a 500 nL loop was used in conjunction with a splitter. Split ratios used were between 5 1 and 50 1, depending on sample concentration and the chosen linear velocity, while the timed injection duration ranged from 50 ms to 1 s. We found that the variation of both the split ratio and injection time allowed greater control over the... 

In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

FIGURE 7.17 Gas chromatograms obtained by the plasma chip (emission at three different wavelengths) and by the flame ionization detector (FID) for the separation of methanol, CH2C12, and CHC13 [563]. Reprinted with permission from the Royal Society of Chemistry. 

Detectors used to identify DEHP include the electron capture detector (ECD) (Mes et al. 1974 Vessman and Rietz 1974) and the flame ionization detector (FID) (Albro et al. 1984). When unequivocal identification is required, a mass spectrometer (MS) coupled to the GC column might be employed (Ching et al. 1981 a EPA 1986 f Hillman et al. 1975 Sjoberg and Bondesson 1985). Analytical methods for the determination of DEHP in various biological fluids and tissues are summarized inTable7-l. 

There are other major problems with peak assignment on the basis of the areas. These problems relate to the reproducibility of peak area measurements under widely varying conditions. Ideally, the area of a peak remains constant even if its capacity factor varies. However, varying the conditions may affect the peak areas. If the column temperature is changed in GC, then the flow rate may be affected. Peak areas will change (by a constant factor) if concentration-sensitive detectors such as the hot wire detector (H WD katharo-meter) are used, but not with mass flow sensitive detectors (such as the flame ionization detector, FID). 

Virtually every conceivable means of detecting gases and vapors has been exploited in designing GC detectors, and over one hundred have been described. The two most popular ones, the thermal conductivity detector (TCD) and the flame ionization detector (FID), will be described in some detail. They are classified (according to the criteria in Chapter 7) and compared in Table 6. 

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