Electron-capture detection, separation

Fitzpatrick, F.A., Stringfellow, D.A., Maclouf, 3. and Rigaud, M. Glass capillary gas chromatography with electron capture detection. Separation of Prostaglandins. 3. Chrom. 177 51-60, 1979... 

The special problems for vaUdation presented by chiral separations can be even more burdensome for gc because most methods of detection (eg, flame ionization detection or electron capture detection) in gc destroy the sample. Even when nondestmctive detection (eg, thermal conductivity) is used, individual peak collection is generally more difficult than in Ic or tic. Thus, off-line chiroptical analysis is not usually an option. Eortunately, gc can be readily coupled to a mass spectrometer and is routinely used to vaUdate a chiral separation. 

The first bioanalytical application of LC-GC was presented by Grob et al. (119). These authors proposed this coupled system for the determination of diethylstilbe-strol in urine as a replacement for GC-MS. After hydrolysis, clean-up by solid-phase extraction and derivatization by pentafluorobenzyl bromide, the extract was separated with normal-phase LC by using cyclohexane/1 % tetrahydrofuran (THE) at a flow-rate of 260 p.l/min as the mobile phase. The result of LC-UV analysis of a urine sample and GC with electron-capture detection (ECD) of the LC fraction are shown in Ligures 11.8(a) and (b), respectively. The practical detection limits varied between about 0.1 and 0.3 ppb, depending on the urine being analysed. By use of... 

Several methods are available for the analysis of trichloroethylene in biological media. The method of choice depends on the nature of the sample matrix cost of analysis required precision, accuracy, and detection limit and turnaround time of the method. The main analytical method used to analyze for the presence of trichloroethylene and its metabolites, trichloroethanol and TCA, in biological samples is separation by gas chromatography (GC) combined with detection by mass spectrometry (MS) or electron capture detection (ECD). Trichloroethylene and/or its metabolites have been detected in exhaled air, blood, urine, breast milk, and tissues. Details on sample preparation, analytical method, and sensitivity and accuracy of selected methods are provided in Table 6-1. 

The most common final separation techniques used for agrochemicals are GC and LC. A variety of detection methods are used for GC such as electron capture detection (BCD), nitrogen-phosphorus detection (NPD), flame photometric detection (FPD) and mass spectrometry (MS). For LC, typical detection methods are ultraviolet (UV) detection, fluorescence detection or, increasingly, different types of MS. The excellent selectivity and sensitivity of LC/MS/MS instruments results in simplified analytical methodology (e.g., less cleanup, smaller sample weight and smaller aliquots of the extract). As a result, this state-of-the-art technique is becoming the detection method of choice in many residue analytical laboratories. 

Niclosamide, formulated as the ethanolamine salt, was determined in formulations and the impact of residues on the environment assessed [82], Efficient (>85%) phase-transfer, N, Odimethylation of niclosamide and the synthesized 5-deschloro analog internal standard, followed by gas-liquid chromatography separation and electron-capture detection, permitted the determination of as little as 10 ppb analyte in fortified, stagnant water. 

The gas chromatographic determination of isomers of dinitrotoluene in seawater has been described by Flashimoto and co-workers [296,297]. These authors describe the complete separation of six dinitrotoluene isomers using gas chromatography with support-coated open tubular glass capillary columns and electron-capture detection. The method was applied to the qualitative and quantitative analyses of trace levels of isomers in seawater and the results were found to be satisfactory, with no need for further clean-up procedures. 

Ealy [ 75 ] also used conversion to alkyl mercury iodides for the gas chromatographic determination of organomercury compounds in benzene extracts of water. The iodides were then determined by gas chromatograph of the benzene extract on a glass column packed with 5% of cyclohexane-succinate on Anakron ABS (70-80 mesh) and operated at 200 °C with nitrogen (56 ml min-1) as carrier gas and electron capture detection. Good separation of chromatographic peaks was obtained for the mercury compounds as either chlorides, bromides, or iodides. The extraction recoveries were monitored by the use of alkylmer-cury compounds labelled with 203 Hg. 

Gas chromatography can also be used with other detection systems, e.g. electron-capture detection (GC-ECD) and flame-ionization detection (GC-FID). The optimization will have been carried out during validation studies. The peak separation criterion is the same as that given for LC above. Once again, cochromatography can be used for confirmation. 

Kawahara FH, Dunn JR, Fiutem RA, et al. 1982. Determination of benzidines by gas chromatographic separation of derivatives with electron capture detection. Anal Chim Acta 138 207-220. 

If the combination of a high-resolution separation system with a physicochemical detection principle is specific to properties of many harmful (toxic) chemicals, then a straightforward approach may be possible (27). Electron capture detection (ECD) registers electronegative compounds. Since the early 1960s, Lovelock (28) has pointed at the possible link between the ability of a substance to capture electrons and its biological action. 

For isohexide nitrates gas-liquid chromatographic separation methods using OF-1 (Ref. 52), OV-101 (Refs. 53-59), OV-17 (Ref 60), or OV-210 (Ref 61) columns have been reported. Most of the methods described are used to detect traces of nitrates, as well as of their metabolites, in urine and plasma probes.62-69 All known isohexide nitrates can be measured in mixtures by using a DB-5 capillary column.70-71 Electron-capture detection was applied for the g.l.c. determination of isosorbide dinitrate71 and its metabo-lites.7,b... 

Because of evidence that BCME is carcinogenic even at very low levels in the atmosphere, current studies of its analysis are concentrating on extending the detection limit to even lower levels. Because of its chlorine content, BCME can be measured with extreme sensitivity by electron capture detection after gas chromatographic separation of this analyte. Efforts are underway by Dr. Robert Sievers (University of Colorado, Boulder) to improve collection methods that meet the criteria of (1) highly efficient collection of BCME at sub-ppb levels in air, (2) no loss of analyte from hydrolysis resulting from atmospheric humidity, and (3) rapid, efficient, nondestructive desorption of analyte from the collection medium. 

Polychlorinated biphenyls (PCBs) have been known for sometime as persistent pollutants, which can be readily bioaccumulated through the food chain causing well-documented toxic effects in number of species including humans [191]. Consequently, PCBs are commonly routinely monitored as potential industrial pollutants. Due to their environmental persistence and toxicity, detection limits in the ng ml-1 region are generally required. Consequently, such work has generally required solvent or solid extraction and concentration steps prior to separation by GC in conjunction with electron capture detection, or mass... 

Figure 13.2 MDGC-ECD chromatograms of PCB fractions from sediment samples, demonstrating the separation of the enantiomers of (a) PCB 95, (b) PCB 132, and (c) PCB 149 non-labelled peaks were not identified. Reprinted from Journal of Chromatography, A 723, A. Glausch et al., Enantioselective analysis of chiral polychlorinated biphenyls in sediment samples by multidimensional gas chromatography-electron-capture detection after steam distillation-solvent extraction and sulfur removal , pp. 399-404, copyright 1996, with permission from Elsevier Science.

GC has been used extensively for the separation and determination of volatile organic molecules, and most aspects of this application area are fully documented in monographs on this technique. In the inorganic trace analysis area, however, fewer species possess the required volatility, and applications tend to be limited to the separation of volatile species of low molecular weight (such as methyl derivatives of As, Se, Sn, Hg) and the separation of semi-volatile organo-metals, metal halides, metal hydrides, metal carbonyls and metal chelates. For organo-metal species, the type of detection system required varies with the nature of the analyte, and the options include electron capture detection, flame photometric detection (sometimes ICP), AAS and MS. 

Basic techniques for speciation analysis are typically composed of a succession of analytical steps, e.g. extraction either with organic solvents (e.g. toluene, dichloromethane) or different acids (e.g. acetic or hydrochloric acid), derivatisa-tion procedures (e.g. hydride generation, Grignard reactions), separation (gas chromatography (GC) or high-performance liquid chromatography (HPLC)), and detection by a wide variety of methods, e.g. atomic absorption spectrometry (AAS), mass spectrometry (MS), flame photometric detection (FPD), electron capture detection (ECD), etc. Each of these steps includes specific sources of error which have to be evaluated. 

Hagenmaier et al. [59] have described a method for the quantitative gas chromatographic determination of volatile halogenated hydrocarbons in lake water samples. Sample enrichment is effected by liquid-liquid extraction with pentane, followed by separation on a capillary gas-liquid chromatographic column, with electron capture detection. A 1 25 pentane-water ratio was employed in conjunction with a standard solution of a reference compound (1-bromobutane) for estimating extraction... 

For the determination of nitrate, use a 1 dram vial with a polyethylene stopper (Kimble No. 60975-L) as a reaction vessel. Introduce a 0.20ml aliquot of aqueous sample into the vial, followed by 1.0ml of thiophen free benzene. Catalyse the reaction by addition of 1.0ml of concentrated sulphuric acid. Shake the vial for lOmin. Remove the benzene layer immediately from the reaction vial with a Pasteur pipette, place it in a separate vial and analyse by gas chromatography with electron capture detection for the nitrobenzene concentration generated. Treat standard solutions of potassium nitrate in the same manner to generate a standard calibration plot relating nitrobenzene concentration to peak height. If higher precision is desired (approximately 4% relative standard deviation), add 2,5-dimethylnitrobenzene to the benzene prior to reaction. 

The oxidatively cleavable linker/tag complex has broader application and is introduced into the polymeric support by a rhodium-catalyzed carbene insertion. The tags are liberated from the solid support by treatment with ceric ammonium nitrate. For analysis the released tags are first silylated and then separated by gas chromatography. Since the tags are electrophoric, they can be analyzed in subpicomole quantity by electron capture detection. The structure of the compound is deduced from the resulting chromatogram. 

The traditional method for extracting and separating Hg species from organic matrices for electron capture detection (ECD) is well established and commonly known as the Westoo method since 1966-1967 [22, 23]. It is mostly used for the determination of Me-Hg. Most of the protocols used to date are derived from this method and the general scheme can be summarized as follows ... 

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