Detection methods electron capture detector

Capillary columns may provide the best method for the separation of phenols prior to their quantification (Eichelberger et al. 1983 Shafer et al. 1981 Sithole et al. 1986). Of the various methods available for detection, the two commonly used methods that are most sensitive are mass spectrometry and flame ionization detection. Although electron capture detectors provide good sensitivities for higher chlorine-substituted phenols, they are poor for phenol itself (Sithole et al. 1986). The best method for the quantification of phenol may be mass spectrometric detection in the selected ion mode, but the loss of qualitative information may be significant (Eichelberger et al. 1983). 

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). 

Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

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... 

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,... 

Elimination of wet chemical sample preparation enables a complete analysis to be performed and data to be quickly analyzed. The detection limits are in the low part-per-million range using mass spectrometric detection. Alternatively, detection of compounds can be achieved by all common gas chromatography detectors (flame ionization detector, electron capture detector and flame photometric detector), and detection limits are determined by the method of detection employed. 

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)... 

The more advanced instrumental methods of analysis, including GC, for the detection and identification of expls are presented (Ref 90) Pyrolysis of expls in tandem with GC/MS was used for the identification of contaminant expls in the environment (Ref 108). Isomer vapor impurities of TNT were characterized by GC-electron capture detector and mass spectrometry (Ref 61). Volatile impurities in TNT and Comp B were analyzed using a GC/MS the GC was equipped with electron capture and flame ionization detectors (Ref 79). The vapors evolved from mines, TNT, acetone, toluene, cyclohexanone and an organosilicon, were analyzed by GC/MS (Ref 78). Red water produced by the sellite purification of crude TNT was analyzed by GC/MS for potentially useful organic compds, 2,4-dinitrotoluene, 3- and 4-sulfonic acids (Ref 124). Various reports were surveyed to determine which methods, including GC/MS, are potential candidates for detection of traces of TNT vapors emitted from land mines factors influencing transportability of TNT vapors thru soil to soil/air interface are dis-... 

Most published methods are for analysis of crops and soil residues of the intact acaricides. Extraction has been done by stripping, blender or soxhlet. Extraction solvents have included petroleum ether, benzene, carbon tetrachloride, acetonitrile, diethyl ether, methanol and hexane/acetone. Clean-up steps have em -ployed liquid/liquid partitioning and adsorption on activated charcoal, activated charcoal/Florisil, Florisil, alumina and silica gel. Burke (14) reported that CB is not completely recovered from Florisil. Horn and coworkers (7) found that no clean-up was necessary when analyzing dog urine for CB using a Schecter-Haller procedure. For detection of residues, the colorimetric and UV methods have been replaced by gas chromatographic methods employing microcoulometric or electron capture detectors. 

Difficulties have been observed in the preservation of samples for speciation of chromium. Chromium speciation in seawater was determined on board ship shortly after samples had been collected (Abollino et at., 1991). Some samples were frozen, and analysed later in a laboratory. However, significantly lower concentrations of Crvl were observed in these latter samples. Thus, sea-going analytical methods for the determination of Crm and total chromium are of particular importance (Mugo and Orians, 1993). The volatile trifluoroacetyl-acetone derivative of Crm was formed and then concentrated by extraction into toluene. Chromium was determined by means of a gas chromatograph equipped with an electron capture detector. Total chromium was determined as Cr111 after reduction. The detection limits were 0.062 and 0.255 nmol dm 3 total chromium. A useful method was described for sampling natural water in the field, and for the preservation of Crm and Crvl species for subsequent analyses in a laboratory (Cox and McLeod, 1992). Water samples were drawn through small columns packed with activated alumina, which had been prepared previously. Chromium species were retained on the columns. 

Various alkyl and aryltin compounds were determined in aquatic matrices, namely sediments, biota and water by means of gas chromatographic methods. In this work, comparisons of single or dual flame photometric detectors and electron capture detectors were reported (Tolosa et al., 1991). Sample preparations included acid digestion, extraction, formation of methyl derivatives and clean-up with alumina prior to gas chromatographic analysis. With the electron capture detector, cold on-column injection of organo-tin chlorides was studied. The conclusion was that a single or dual flame photometric detector equipped with a 600 nm interference filter yielded the best performance for determinations of tin species as methyl derivatives. Detection limits for the method using flame... 

Traces of explosives are commonly present in very low levels in samples that are analysed, so it is important to take sensitivity into account when designing detectors for explosive detection. As a rough rule of thumb , Nambayah and Quickenden [38] reported that a method suitable for direct explosive vapour detection should be able to detect explosive concentrations at less than 1 ng/L. They made an exhaustive study of the lowest experimental detection limits achieved with various analytical techniques reported in the literature on traces of explosive, and they informed that headspace GC-electron capture detector (ECD) followed by immunosensor techniques achieves the lowest detection limits (from 0.07 to 20 ng/L). 

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