Electron capture detector, hydrocarbon

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. 

Figure 3. Capillary gas chromatogram of chlorinated hydrocarbons in a blood sample, magnesium plant worker using electron capture detector. Peak identities 1, pentachlorobenzene 2, hexachlorobenzene 3, heptachlorostyrene 4, octachloro-styrene 5, unknown 6, p,p -DDE (p is probably PCB isomers).

Most manuals supplied by chromatograph manufacturers provide instructions for proper maintenance and troubleshooting. When checking the detector, it is advisable to use a sample that will not be adsorbed by the column, such as a hydrocarbon. For electron capture detectors a suitable test substance would be a halo-carbon, rather than a sensitive chlorinated pesticide. 

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. 

The column used was a SPB-608 fused silica capillary column, 30 m x 0.53 mm ID. with a 0.5 pm film of stationary phase. It was programmed from 50°C at l°/min to 150°C and then at 8°/min to 260°C. Helium was used as the carrier gas at a flow rate of 5 ml/min and the sample consisted of 0.6 pi of a solution of the pollutants in ndecane. The mass of each pollutant present was about 120 pg. The electron capture detector is also used extensively for monitoring the separation of polychlorinated hydrocarbons and in particular the herbicides. 

The FPD does not respond to nitrogen or halogens the AFD does, but much less so than to phosphorus. In those areas, the competition comes primarily from other types of detectors. These are the micro-coulometric and the electrolytic conductivity detectors in the nitrogen and halogen areas and, of course, the electron capture detector for chlorinated hydrocarbons and other electron-attaching compounds. 

One day, Jim (Lovelock) poked a hole in one of the tritium foils and the plane parallel detector was ready to test.. .. A mixture of components containing different functional groups along with 2 or 3 hydrocarbons was prepared and injected onto a capillary column. Positive deflections were recorded for the hydrocarbons and a series of negative deflections for the ketones, aldehydes, alcohols and particularly the halogenated substances. The electron capture detector came into being with full force. 

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... 

The compounds are representatives for different types of pollutants. 11 halogenated aliphatic hydrocarbons are determined using capillary GC equipped with electron capture detector. The remaining pollutants, representing both basic/neutral and acidic compounds are determined using GC/MS combined with an automated search computer program. 

The electron capture detector is extremely sensitive to certain molecules, such as alkyl halides, conjugated carbonyls, nitriles, nitrates and organometals. It is virtually insensitive to hydrocarbons, alcohols and ketones. In other words, the detector selectively responds to molecules containing electronegative atoms, since these atoms easily attach or attract an electron and thus produce an electrical signal. Quantities of halides at picogram (pg 10 12 gm) levels can be detected by the electron capture detector. 

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. 

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