Flame detectors, description

Figure 10.4 Schematic representation of the multidimensional GC-IRMS system developed by Nitz et al. (27) PRl and PR2, pressure regulators SV1-SV4, solenoid valves NV— and NV-I-, needle valves FID1-FID3, flame-ionization detectors. Reprinted from Journal of High Resolution Chromatography, 15, S. Nitz et al, Multidimensional gas cliro-matography-isotope ratio mass specti ometiy, (MDGC-IRMS). Pait A system description and teclinical requirements , pp. 387-391, 1992, with permission from Wiley-VCFI.

The detector. The function of the detector, which is situated at the exit of the separation column, is to sense and measure the small amounts of the separated components present in the carrier gas stream leaving the column. The output from the detector is fed to a recorder which produces a pen-trace called a chromatogram (Fig. 9.1fr). The choice of detector will depend on factors such as the concentration level to be measured and the nature of the separated components. The detectors most widely used in gas chromatography are the thermal conductivity, flame-ionisation and electron-capture detectors, and a brief description of these will be given. For more detailed descriptions of these and other detectors more specialised texts should be consulted.67 69... 

Many sophisticated analytical techniques have been used to deal with these complex mixtures.5,45,46 A detailed description is not possible here, but it can be noted that GLC, often coupled with mass spectrometry (MS), is a major workhorse. Several other GLC detectors are available for use with sulfur compounds including flame photometer detector (FPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED).47 Multidimensional GLC (MDGC) with SCD detection has been used48 as has HPLC.49 In some cases, sniffer ports are provided for the human nose on GLC equipment. 

The identification of individual classes of fatty acids has relied on the use of gas chromatography (GC), equipped with a flame ionization detector. Lipids are sapoiufled after extraction and the fatty acids converted to methyl esters. The fatty acid methyl esters (FAME) are separated using GC. The use of standards ahowed for the identification of individual species of fatty acids based on retention time. The method is quite sensitive and permits the quantification of fatty acid species. The mode of detection has been enhanced with the use of mass spectrometry (MS), which allows for the detection and quantification of unknowns, thus increasing the utility of these methods. The Lipid Library section on the gas chromatography of lipids http //www.lipidlibrary.co.uk/GCJipid/01 intro/index. htm) provides a comprehensive description of these methods. 

Detectors commonly used in GC and specified in the USPP include FID, alkali FID (NPD, TD), BCD, and TCD. A description of these detectors, including their operational principles and relative performance, was presented in a previous volume of this encyclopedia. Various other useful detectors for GC include photoionization (PID), flame photometric (FPD), electrolytic conductivity (BLCD), redox (RCD) and sulfur chemiluminescence (SCD), and helium ionization (HID).[4 1 Table 1 summarizes some of the features of detectors used in GC. 

Dozens of detectors have been developed for use in GC. The two most widely used, low-cost detectors are the thermal conductivity detector (TCD) and the flame ionization detector (FID). For descriptions of other detectors see Grob (1995) in Bibliography. 

A small sample of a suitable hydrocarbon probe is injected into the moist carrier gas and its emergence from the column is measured with a flame ionization detector. The time required for the probe to pass through the column indicates the interaction between the probe and the column packing. By changing the size of the probe sample, a complete adsorption isotherm may be calculated from the GC data. A full description of the apparatus and theory is available (10). 

Since the introduction of the flame photometric detector (FPD) (Brody and Chaney, 1966) and its first apphcation to marine DMS (Lovelock et al., 1972), gas chromatography with flame photometric detection (GC-FPD) has become the standard technique for the determination of dissolved DMS. Among other sulphur-selective detectors, thus far only the sulphur chemiluminescence detector (SCD) (Benner and Stedman, 1989 Shearer, 1992) has also been used for the determination of dissolved DMS (e.g., Ledyard and Dacey, 1994). However, detailed descriptions of this emerging analytical technique are still lacking. In contrast, the popularity of the FPD resulted in the pubUcation of a variety of methods for the determination of oceanic DMS (e.g., Andreae and Barnard, 1983 Leek and Bdgander, 1988 Turner and Liss, 1985), two of which have been compared during an inter-laboratory calibration (Turner et al., 1990). In the following, some principles of DMS determination by GC-FPD will be discussed, before the analytical procedure is described. 

Workstations and robotic systems are very expensive, so inexpensive alternatives such as flow configurations have been developed for automated sample preparation. The earliest flow systems for sample preparation were used for GC determination (with flame ionization detector [FID] or electron capture detector [EGD] detection) of organic compounds, which requires no special extraction or derivatization, in environmental matrices [30-34]. Automated GC-MS systems for the determination of volatiles in water or air [35-38] are the most commonly reported. Detailed descriptions of these systems can be found elsewhere in this book. Few continuous flow systems (CFSs) for the automated pretreatment of biological fluids in combination with GC-MS have been developed to date. The intrinsically discrete nature of the GC-MS sample introduction mechanism makes online coupling to continuous flow systems theoretically incompatible for reasons such as the different types of fluids used (liquid and gas) and the fact that the chromatographic column affords volumes of only 1 to 2 j,l of cleaned-up extract. Therefore, the organic extracts from CFSs have traditionally been collected in glass vials and aliquots for manual transfer to the GC-MS instrument (off-line approach) only in a few cases is an appropriate interface used to link the CFS to the GC-MS instrument (on-line approach). These are the topics dealt with below. 

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