The Electron Capture Detector ECD

The electron capture detector is an essential comp onent in the determination of trace amounts of organochlorines for which its sensitivity is roughly five orders of magnitude higher than for hydrocarbons. For instance, the detection limit for lindane may be as low as 

Response factors of individual chlorobiphenyls on ECDs depend on the number and the positions of the chlorine atoms in the molecule, and also on the analytical instrument Frame, 1997). The BCD response is affected in particular by characteristics of the detection and injection systems, such as temperature, state of contamination and geometry. Considerable differences exist between detector responses of on-colunm and split/splitless injection, in particular the dependence on the amounts injected (i.e., the degree of linearity). It is not surprising, therefore, that apparently conflicting data are cited in the literature Mullin et at., 1984) and also that significant differences have been found for response factors even between carefully prepared standards. 

The use of published data on response factors (c.g., such as the detailed list in Mullin et at. (1984)) for estimating the responses of other CBs e.g., those not available as reference materials) has limited value. The only reliable way to quantitate CBs in samples uses reference materials containing known amounts of the congeners of interest. 

If the FID brought GC into the realm of characterizing dilute solutions of organics, it was the ECD that allowed it to spark a revolution in the understanding of the threat of bioaccumulation in tissue and bioconcentration up food chains, of lipophilic, persistent organic pollutants (POPs) in the environment. The classic example of this was the discovery of the threat posed by the organochlorine pesticide DDT, and its subsequent banning. 

Lovelock (subsequently famous as the founder of the Gaia theory of the whole earth as an organism) invented this deceptively simple but exquisitely sensitive and selective detector. It is not too much of a reach to claim that observations which were only made possible by the use of this GC detector set off the revolution in environmental consciousness in the 1960s. 

ECD for Use with Pulsed Electrode Potential ECD for Use with Constant Electrode Potential 

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Electron Capture Detector In the electron capture detector (ECD), a beta emitter such as tritium or 63Ni is used to ionize the carrier gas. Electrons from the ionization migrate to the anode and produce a steady current. If the GC effluent contains a compound that can capture electrons, the current is reduced because the resulting negative ions move more slowly than electrons. Thus, the signal measured is the loss of electrical current. The ECD is very sensitive to materials that readily capture electrons. These materials frequently have unsaturation and electronegative substituents. Because the ECD is sensitive to water, the carrier gas must be dry. 

The electron capture detector (ECD) is most frequently used to identify hexachloroethane. A flame ionization detector (FID) may also be used (NIOSH 1994). When unequivocal identification is required, an MS coupled to the GC column may be employed. 

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,... 

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. 

In the domain of gas chromatography the electron capture detector (ECD) enjoys the reputation of being one of the most sensitive as well as selective detectors. However, this valuable detector needs to be handled with a lot of skill and expertise so as to achieve wonderful and dependable results. 

Detector selection was relatively straightforward. Because the electron capture detector (ECD) offered sensitivities for HCCP and HCBD that could not be equaled by any other GC detection system, the ECD was employed for the determination of these two... 

The electron capture detector (ECD) is also a concentration-dependent detector, and like the TCD will give a higher response for a given compound at lower carrier flowrates. Carrier flow-rate must be carefully controlled. Usually a 95% argon - 5% methane mixture is used for carrier gas. Presence of oxygen or water in the carrier gas results in loss of sensitivity and a compression of the linear range. 

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. 

The electron capture detector (ECD) was invented by Lovelock in 1961 and is probably the third most used detector. As its name implies, it is selective for materials that capture electrons—halogen- and nitrogen-containing compounds such as pesticides and unsaturated compounds such as the polynuclear aromatics. It is an ionization detector, but unlike the FID it is a concentration type and a bulk property type detector. As such it is an exception to our generalization that bulk property detectors are not very sensitive. 

TCD) detector or the flame-ionization (FID) detector, which are the two most common detectors in gas chromatography, respond to all (organic) compounds except the carrier gas. On the contrary, a selective detector responds to a range of compounds with a common physical or chemical property. Representatives of the latter group of detectors are the nitrogen-phosphorus detector (NPD), the electron capture detector (ECD), the mass selective detector (MSD) and - last, but not least - the tandem mass spectrometer (MS/MS). 

By use of different detectors trace amounts of metals ranging from 10 to 10 g can be determined (Tables l.l/II, 1.2). Polyfluorinated 3-diketones as ligands give more highly volatile derivatives and above all allow the use of the electron capture detector (ECD) which has low detection limits for fluorinated compounds. Nevertheless decompositions can also appear in this class of substances. In addition thioketones and amino derivatives are used as chelating agents for quantitative gas-chromatography. 

In trace analysis of contaminant substances, one can use specific detectors for certain compounds, such as a nitrogen-phosphorus detector (NPD), thus gaining detection ability for nitrogenated and phosphorylated compounds the electron-capture detector (ECD) shows excellent performance for chlorinated substances and the flame photometric detector (FPD) is the most widely used for sulfur-containing compounds. 

World War I led to Tswett s premature death at age 41 and interrupted the development of chromatography. World War II postponed the further study of gas chromatography until the early 1950s [ 1], It was not until 1960 that gas chromatography was demonstrated, capillary columns were developed, and ionization detectors including the electron capture detector (ECD) were invented and commercialized [6-8],... 

The equilibrium, beam, and photon methods have been used to measure Ea. The electron capture detector (ECD), magnetron (MGN), and swarm equilibrium... 

More than a century ago Thompson determined the mass-to-charge ratio of the electron and established its fundamental nature. It remains the only one of the subatomic particles that has not been subdivided. Simultaneously, Tswett initiated the study of modern chromatography. Fifty years later Lovelock observed that the reaction of molecules with thermal electrons greatly perturbed ionization currents generated by radioactivity in air. This led to the electron capture detector (ECD) and inextricably bound chromatography and the reactions of thermal electrons with molecules. 

The more universal and less sensitive flame-ionization detector (EID) is used much less often than the electron capture detector (ECD), which has exceptional sensitivity to multiply chlorinated compounds. The mass spectrometer-selected-ion-monitoring (MS-SIM) or ion-trap mass spectrometer (ITMS)... 

The electron capture detector (ECD) (Figure 13.8) measures the loss of signal rather than an increase in electrical current. As the nitrogen carrier gas flows... 

The electron capture detector (ECD), sensitive to electronegative elements, has been reported to be very sensitive for sulfur species. One of the disadvantages of this detector, however, is the strong compound dependence of its response. 

At the temperatures and pressures generally used in gas chromatography the common carrier gases employed behave as perfect insulators. In the absence of conduction by the gas molecules themselves, the increased conductivity due to the presence of very few charged species is easily measured, providing the low sample detection limits characteristic of ionization based detectors [259]. Examples of ionization detectors in current use include the flame ionization detector (FID), thermionic ionization detector (TID), photoionization detector (PID), the electron-capture detector (ECD), and the helium ionization detector (HID). Each detector employs a different method of ion production, but in all cases the quantitative basis of detector operation corresponds to the fluctuations of an ion current in the presence of organic vapors. 

Selective detectors can be selective towards elements, structure, or other properties. The flame ionization detector (FID) reacts selectively to substances that are ionized in a hydrogen/air flame (very broad selectivity). It is useful for the analysis of aqueous samples, as the water is not recorded. The electron capture detector (ECD) is selective for halogenated compounds. 

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