Electron capture detector, hydrocarbon analysis

A third type of detector, required for some environmental and biomedical applications, is the electron capture detector (ECD). This detector is especially useful for large halogenated hydrocarbon molecules since it is the only one that has an acceptable sensitivity for such molecules. Thus, it finds special utility in the analysis of halogenated pesticide residues found in environmental and biomedical samples. 

For the analysis of sulfur hexafluoride an electron capture detector must be used. Analysis time for halothane, acetone, ether, cyclopropane, acetylenes, ethane, methane, and sulfur hexafluoride, under normal conditions, will average 8 min. Since the two detectors operate under different conditions samples containing both the hydrocarbons and sulfur hexafluoride are run through the gas chromatograph twice. By simultaneous use of separate ovens, the FID and ECD measurements can be made concurrently. Otherwise, the separate in jections must be made after switching the detectors. 

At this point, the sample is analyzed by gas chromatography (GC), the analytical method of choice for volatile halogenated hydrocarbons. Information on the analysis of these samples by GC is presented in Section 6.2, with a discussion of the advantages and disadvantages of each method. The technique of Antoine et al. (1986) showed a 5% variance on a series of 2 ppb spiked samples, and the analysis had a linear response ranging from 0.5 to 50 ppb. Although infra-red spectrometry has less sensitivity than electron capture detectors (ECD), Hall electroconductivity detectors (HECD), and mass spectrometrlc detectors (MS), it has been used to quantify the levels of... 

Because of the large scale dilution of contaminants in the aquatic matrices, concentrations of many organic pollutants are below the detection limits of standard analytical and sampling methods. Thus, gas chromatography with specific detection methods such as electron capture detector and HPLC has been frequently used for analysis of pesticides and polycyclic aromatic hydrocarbons in water and biological samples. 

The electron capture detector is especially suitable for halogen, sulfur, and nitro-compounds, i.e., compounds that are able to capture electrons. Because of its high selectivity and sensitivity, it is much used for residue analysis, e.g. for volatile halogenated hydrocarbons and plant protection agents. Figure 4-2 shows the design of an ECD. 

Herbicides may be determined by specific extraction, esterification, and gas chromatographic conditions. Aqueous samples are extracted with diethyl ether and then esterified with either diazomethane or pentafluorobenzyl bromide. The derivatives are determined by GC with an electron capture detector. Compound identifications should be supported by GC-MS for the qualitative confirmation. Alkaline hydrolysis and subsequent solvent wash removes many chlorinated hydrocarbons and phthalates that might otherwise interfere with the electron capture analysis. 

Pesticides and Fungicides. Modern pure food regulations require that the food processor be responsible for their finished products. Since so many pesticides and fungicides are used in agriculture, their detection and quantitative analysis are difficult (5, 22). Organophosphorus and chlorinated hydrocarbons are the most common pesticides. When GLC is used for halogens, electron capture or microcoulometric detectors are used for phosphorus, a thermionic flame photometric detector is required. 

Dual Detectors. Most dual detectors are run in parallel, the column effluent being split and run through both of them simultaneously. In GC the technique is known as dual channel GC usually, one of the detectors chosen is universal and the other is highly selective. Figure 6.6 shows the analysis of a commerical gasoline sample with dual detection by flame ionization (FID) and electron capture (ECD). The FID detects all the hydrocarbons, but the ECD is selective for the alkyl lead additives in gasoline and permits their detection without interference from the hydrocarbons. 

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