Electron-capture detector sensitivity

Analysis of Gas Chromatography/Electron Capture Detector Sensitive Substances in Human Serum in Relation to the Determination of lodochlorhydroxyquin Concentration Clin. Chim. Acta 100(1) 75-79 (1980) CA 92 90210V... 

Sulfur hexafluoride may be analyzed chromatographicaHy using a molecular sieve or a Porapak QS column. Using an electron-capture detector, a sensitivity of 10 to lO " ppb is possible (51—53). 

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. 

A very sensitive method for the determination of MCA in surfactants is a gas chromatographic one [249]. The method is based on the derivatization of the sample with ethanol and subsequent extraction of the derived ester with cyclohexane. The acids are identified and qualified gas chromatographically by the use of an electron capture detector and two capillary columns of varying polarities. The detection limit is 0.2 ppm. 

The method of choice for the determination of a- and P-endosulfan in blood, urine, liver, kidney, brain, and adipose tissue is gas chromatography equipped with an electron capture detector (GC/ECD) (Coutselinis et al. 1976 Demeter and Heyndrickx 1979 Demeter et al. 1977 Le Bel and Williams 1986). This is because GC/ECD is relatively inexpensive, simple to operate, and offers a high sensitivity for halogens (Griffith and Blanke 1974). After fractionation of adipose tissue extracts using gel permeation chromatography, detection limits of low-ppb (1.2 ng/g) were achieved for endosulfan and other chlorinated pesticides using GC/ECD (Le Bel and Williams 1986). 

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... 

A technique known as selective electron capti sensitization has been used to increase the response of the BCD weakly electron-capturing compounds [117]. In this mode a standard electron-capture detector is used with a supply of makeup gas doped with a specific sensitizing reagent such as oxygen nitrous oxide. In this way the BCD functions as an ion-aoleculSj... 

The relative response of the electron-capture detector to some haloalkylacyl derivatives is sumi rized in Table 8.17 [451]. In general terms, the monochloroacetyl and chlorodifluoroacetyl derivatives provide a greater response than the trifluoroacetyl derivatives. Increasing the fluorocarbon chain length of the fluorocarbonacyl derivatives increases t ir electron-capture detector response without inconveniently increasing their retention times. The heptafluorobutyryl and pentafluoropropionyl derivatives are considered to be the best compromise between detector sensitivity and volatility for most applications. 

Cyanide and thiocyanate anions in aqueous solution can be determined as cyanogen bromide after reaction with bromine [686]. The thiocyanate anion can be quantitatively determined in the presence of cyanide by adding an excess of formaldehyde solution to the sample, which converts the cyanide ion to the unreactive cyanohydrin. The detection limits for the cyanide and thiocyanate anions were less than 0.01 ppm with an electron-capture detector. Iodine in acid solution reacts with acetone to form monoiodoacetone, which can be detected at high sensitivity with an electron-capture detector [687]. The reaction is specific for iodine, iodide being determined after oxidation with iodate. The nitrate anion can be determined in aqueous solution after conversion to nitrobenzene by reaction with benzene in the presence of sulfuric acid [688,689]. The detection limit for the nitrate anion was less than 0.1 ppm. The nitrite anion can be determined after oxidation to nitrate with potassium permanganate. Nitrite can be determined directly by alkylation with an alkaline solution of pentafluorobenzyl bromide [690]. The yield of derivative was about 80t.with a detection limit of 0.46 ng in 0.1 ml of aqueous sample. Pentafluorobenzyl p-toluenesulfonate has been used to derivatize carboxylate and phenolate anions and to simultaneously derivatize bromide, iodide, cyanide, thiocyanate, nitrite, nitrate and sulfide in a two-phase system using tetrapentylammonium cWoride as a phase transfer catalyst [691]. Detection limits wer Hi the ppm range. 

Detectors are composed of a sensor and associated electronics. Design and performance of any detector depends heavily on the column and chromatographic system with which it is associated. Because of the complexity of many mixtures analysed and the limitation in regard to resolution, despite the use of high-resolution capillary columns and multicolumn systems, specific detectors are frequently necessary to gain selectivity and simplify the separation system. Many detectors have been developed with sensitivities toward specific elements or certain functional groups in molecules. Those detectors that exhibit the highest sensitivity are often very specific in response, e.g. the electron capture detector in GC or the fluorescence detector in LC. Because... 

Only when the very contamination-sensitive electron-capture detector is used is it necessary to provide separate gas streams, one for the reaction and stripping part of the system, the other for the carrier gas stream of the column and detector. Otherwise, the same gas stream can be used to strip the hydrides from solution and carry them into the detector, which greatly simplifies the apparatus. This is of considerable significance, as each additional surface and joint in the apparatus increases the possibility of irreversible adsorption of the sensitive hydrides, and thus is a potential contributor to analytical error. The... 

The electron-capture detector was originally found to be a sensitive detector for the methylarsines [716]. After improvements of the atomic absorption detectors had been made (especially concerning adsorptive losses and peak shapes of the methylarsines), it was found that this detector could be used to replace the electron-capture detector, which because of its lack of specificity and its sensitivity to contamination and changes in operating conditions was very inconvenient to work with. 

Most often the sterols have been collected by liquid-liquid extraction using petroleum ether and ethyl acetate [408], chloroform and methanol [409], -hexane [410,411] or chloroform [412,413]. After concentration, gas chromatography was generally used for the final separation and determination, although thin-layer chromatography has also been employed. The extra sensitivity of the electron capture detector could be used by reacting the concentrated sterols with bromomethyldimethylchlorosilane (BMDS) before separation and measurement [414],... 

The heptafluorobutyric anhydride derivative of pseudoephedrine and electron capture detector have been used to enhance the sensitivity of the gas chromatographic method. Lin and co-workers36 and Cummins and Fourier37 extracted basefied urine or serum with benzene. Heptafluorobutyric anhydride is added to the benzene extract. The heptafluoro-ibutyric anhydride derivative extracted was chromatographed... 

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,... 

One of the most commonly used detection systems in a gas chromatography laboratory is the electron capture detector. The first paper [25] to be published demonstrating the use of an electron capture detector with supercritical fluid chromatography showed that with supercritical fluid chromatography sensitivity to about 50pg minimum detection limit on column was obtainable. 

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. 

Electron capture detectors are extremely sensitive (1 X 10 12 mol) but are specific for electrophilic compounds. However, they can be used in parallel with flame ionization detectors to identify specific peaks in a chromatogram. 

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