Electron Capture Detector ECD

The electron capture detector principle is based on the phenomenon that electronegative specifies (CX) can react with thermal electrons to form 

In the early stages, ECD used a constant and relatively low (10 - 20 V) voltage apphed to the detector cell and the variations of the current during the analyte elution were recorded. A recent design apphes pulse frequency (which is constantly adjusted) to maintain the cell current at a constant value. This method avoid reactions of analyte molecules with high energy electrons, and offer a much wider dynamic linear range than the constant frequency ECD. 

It should be understood that there are two types of detectors those which produce unequal signals from equal concentrations of different substances in the mobile phase are specific. If this only applies to substances incorporating a particular element, a particular functional group or with some other outstanding property, the detector is selective. 

When operating the FID, care must be taken that the flows of H2, N2 and air are set at a constant rate precisely according to the specifications of the equipment manufacturer. The gases must be dry and of high purity. The FID is outstanding for its very high sensitivity and wide range of 

Virtually all modern electron capture detectors operate according to the pulse-modulated constant-current technique (PMCC). The ionization source is a Ni foil (B -emitter), which is attached to the inside of the 

Ar/CHz/i) flows through the cell, the gas becomes ionized. The slow electrons formed flow to the anode, thus generating a current. 

If negative pulses with a frequency of fQ are applied to the foil, the electrons are able to reach the collector only during a negative pulse period. In this way, the pulse frequency fo determines the residual current of the ECD. If an electron-absorbing substance S enters the detector cell, the electron concentration falls. This loss is compensated by an increase in pulse frequency to fg, so that the residual current re- 

Electron capture is more effective, the slower the electrons move. For this reason, sensitive ECDs are operated using a pulsed DC voltage. By changing the pulse frequency, the current generated by the electrons is kept constant. The pulse frequency thus becomes the actual detector signal. 

Selective, mass-flow-dependent-detector Non-destructive, can be stacked with FID, NPD 

The ECD reacts with all electronegative elements and functional groups, such as -F, -Cl, -Br, -OCH3 and -NO2 with a high response. All hydrocarbons (generally the matrix) remain transparent, although present. 

Radioactive radiator, therefore local handling authorization necessary. 

Volatile halogenated hydrocarbons, freons, nitrous oxides. Substances with low halogen contents (0, CI2) only have a low response (Table 2.38). 

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Ionisation detectors. An important characteristic of the common carrier gases is that they behave as perfect insulators at normal temperatures and pressures. The increased conductivity due to the presence of a few charged molecules in the effluent from the column thus provides the high sensitivity which is a feature of the ionisation based detectors. Ionisation detectors in current use include the flame ionisation detector (FID), thermionic ionisation detector (TID), photoionisation detector (PID) and electron capture detector (ECD) each, of course, employing a different method to generate an ion current. The two most widely used ionisation detectors are, however, the FID and ECD and these are described below. 

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 structure-selective, electron-capture detector (ECD) is the second aost widely used ionization detector [115-118]. It owes Buch of its popularity to its unsurpassed sensitivity to a wide range of toxic and biologically active coapounds. Consequently, it is widely used in trace analysis for the detemination of pesticides, herbicides and industrial chemicals in the... 

The CGC analysis of the volatile degradation products were performed using a Perkin-Elmer Sigma 2000 capillary gas chromatograph. The column used was either a fused silica 0.25 micron, bonded methyl silicone (10 m, 0.25 mm I.D.) or a methyl/5% phenyl silicone (15 m 0.25 mm I.D.) bonded phase. The carrier gas was helium and the capillary column head pressure was maintained at 20 psi. The make-up gas for the pulsed electron capture detector (ECD) was 95% Ar/5% methane supplied at a flow rate of 60 ml/min. 

The heptafluorobutyrate derivative was selected for gas chromatographic separation, using electron capture detector (ECD), in order to enable the detection of ultramicro quantities43. The interest in the analysis of natural and synthetic hormones in very small concentrations enhanced the development of the GC method, in comparison with the UV study.44... 

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,... 

There are four main types of detectors used in GC thermal conductivity detector (TCD), also called a hot wire detector, flame ionization detector (FID), electron capture detector (ECD), and quadruple mass spectrometer (MS)... 

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. 

The best sensitivity for 1,2-dibromoethane quantification is obtained by either electron capture detector (ECD) or Hall electrolytic conductivity detector (HECD) in the halide detection mode, since these detectors are relatively insensitive to nonhalogenated species and very sensitive to halogenated species. Another common detection device is a mass spectrometer (MS) connected to a GC. The GC/MS combination provides unequivocal identification of 1,2-dibromoethane in samples containing multiple components having similar GC elution characteristics (see Table 6-2). To date, GC equipped with either ECD or HECD has provided the greatest sensitivity for detecting... 

Another possible detector for well defined samples where the possibility of unexpected components is low is the Ni electron capture detector (ECD). Application of this with the purge and trap detector is tedious because the ECD is so sensitive and susceptible to many trace interfering substances. Also temperature programming is... 

P33 Analyses wereper/ormed on a gas chromatograph equipped with an electron capture detector (ECD) and a gas chromatograph coupled to a mass-selective detector working in mass spectrometry-mass spectrometry (MS-MS) mode, to achieve better limits of detection and selectivity. The proposed method yields high sensitivity, good linearity, precision, and accuracy. (From Dellinger et ah, 2001)... 

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... 

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