about choosing the right detector settings, summarizing much of the material from this section has been published by Hinshaw [11] .
FLAME IONIZATION DETECTOR ( FID )
The FID is the most widely used GC detector, and is an example of the ioniza-tion detectors invented specifi cally for GC. The column effl uent is burned in a small oxy - hydrogen fl ame producing some ions in the process. These ions are collected and form a small current that becomes the signal. When no sample is being burned, there should be little ionization, the small current (10 − 14 a) arising from impurities in the hydrogen and air supplies. Thus, the FID is a specifi c property - type detector with characteristic high sensitivity.
A typical FID design is shown in Fig. 7.8 . The column effl uent is mixed with hydrogen and led to a small burner tip that is surrounded by a high fl ow of air to support combustion. An igniter is provided for remote lighting of the fl ame. The collector electrode is biased about +300 V relative to the fl ame tip and the collected current is amplifi ed by a high impedance circuit. Since water is produced in the combustion process, the detector must be heated to at least 125 ° C to prevent condensation of water and high boiling samples. Most FIDs are run at 250 ° C or hotter.
The exact mechanism of fl ame ionization is still not known. For the early theories, see Sternberg et al. [12] and later discussion by Sevcik et al. [13] . The
Fig. 7.8. Schematic of an FID. Courtesy of Perkin - Elmer.
FID responds to all organic compounds that burn in the oxy - hydrogen fl ame.
The signal is approximately proportional to the carbon content, giving rise to the so - called equal per carbon rule, a constant response factor that may be due to the conversion of all carbon atoms in an organic solute to methane in the FID combustion process [14] . Thus, all hydrocarbons should exhibit the same response, per carbon atom. When heteroatoms like oxygen or nitrogen are present, however, the factor decreases. Relative response values are often tabulated as effective carbon numbers , ECN; for example, methane has a value of 1.0, ethane has a value of 2.0, and so on. Table 7.2 lists experimental and theoretical ECN values for some simple organic compounds [15] . Clearly
TABLE 7.2 FID Effective Carbon Numbers (Relative by permission of Preston Publications, A Division of Preston Industries, Inc.
FLAME IONIZATION DETECTOR (FID) 117
Fig. 7.9. Effect of hydrogen fl ow rate on FID response. Courtesy of Perkin - Elmer.
From Miller, J. M., Chromatography: Concepts and Contrasts , 2nd ed., John Wiley &
Sons, Hoboken, NJ, 2005, p. 159. Reproduced courtesy of John Wiley & Sons, Inc.
response factors are necessary for good quantitative analysis (see Chapter 8 for some weight response values).
For effi cient operation, the gases (hydrogen and air) must be pure and free of organic material that would increase the background ionization. Their fl ow rates need to be optimized for the particular detector design (and to a lesser extent, the particular analyte). As shown in Fig. 7.9 , the fl ow rate of hydrogen goes through a maximum sensitivity for each carrier gas fl ow rate, the optimum occurring at about the column fl ow rate. For open tubular columns that have fl ows around 1 mL/min, make - up gas is added to the carrier gas to bring the total up to about 30 mL/min.
Hydrogen can be used as the carrier gas, but changes in gas fl ows (a separate source of hydrogen is still required) and detector designs are required [16] in addition to the safety precautions that must be taken.
The fl ow rate of air is much less critical, and a value of 300 – 400 mL/min is suffi cient for most detectors, as shown in Fig. 7.10 .
Compounds not containing organic carbon do not burn and are not detected.
The most important ones are listed in Table 7.3 . Most signifi cant among those listed is water, a compound that often produces badly tailed peaks. The absence of a peak for water permits the FID to be used for analysis of samples that contain water since it does not interfere in the chromatogram. * Typical appli-cations include organic contaminants in water, wine and other alcoholic bever-ages, and food products.
The list on page 118 summarizes the characteristics of the FID. Its advan-tages are good sensitivity, a large linearity, simplicity, ruggedness, and adapt-ability to all sizes of columns.
* Water may hydrolyze some of the polyester and polyethylene glycol liquid phases, but it has little to no effect on silicone polymers.
Flame Ionization Detector ( FID ) Characteristics 1. MDQ — 10 − 11 g ( ∼ 50 ppb)
2. Response — organic compounds only, no fi xed gases or water 3. Linearity — 10 6 — excellent
4. Stability — excellent, little effect of fl ow or temperature changes 5. Temperature limit — 400 ° C
6. Carrier gas — nitrogen or helium
Fig. 7.10. Effect of air fl ow rate on FID response. Courtesy of Perkin - Elmer. From Miller, J. M., Chromatography: Concepts and Contrasts , 2nd ed., John Wiley & Sons, Hoboken, NJ, 2005, p. 160. Reproduced courtesy of John Wiley & Sons, Inc.
TABLE 7.3 Compounds Giving Little or No Response in the Flame Ionization Detector
He CS 2 NH 3
Ar COS CO
Kr H 2 S CO 2
Ne SO 2 H 2 O
Xe NO SiCl 4
O 2 N 2 O SiHCl 3
N 2 NO 2 SiF 4