Concentric electron-capture detectors

The detector. The function of the detector, which is situated at the exit of the separation column, is to sense and measure the small amounts of the separated components present in the carrier gas stream leaving the column. The output from the detector is fed to a recorder which produces a pen-trace called a chromatogram (Fig. 9.1fr). The choice of detector will depend on factors such as the concentration level to be measured and the nature of the separated components. The detectors most widely used in gas chromatography are the thermal conductivity, flame-ionisation and electron-capture detectors, and a brief description of these will be given. For more detailed descriptions of these and other detectors more specialised texts should be consulted.67 69... 

Decomposition products were captured in two traps immersed in dry ice followed by a third trap containing 5 ml of benzene. After 1 hour, the boat, tube, and traps were rinsed with benzene. The benzene solution was analyzed by gas chromatography using an electron capture detector to determine the concentration of unreacted 2,3,7,8-tetrachloro-dibenzo-p-dioxin. 

Selenium(IV) reacts with substituted 1,2-diaminobenzene or 2,3-diamlnonaphthalene in acidic solution to form stable cyclic derivatives which can t>e extracted into an organic solvent and analyzed by gas or liquid chromatography [682,683]. With chloro-, bromo-, or nitro-substituents the plazselenols can be determined with an electron-capture detector at the low picogram level. Se(VI) does not form piazselenol derivatives so the reaction with diaminobenzene can be used to determine the concentration of Se(IV) and Se(VI). Selenium(VI) may be redu to Se(IV) 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 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... 

PGDN was generated from a sample of Otto Fuel II by blowing air across a Pyrex reservoir of the compound to the return air duct of the air conditioner. Eighty percent of the air was recirculated. The concentration of PGDN in the air was monitored continuously by an infrared spectrophotometer and by a gas chromatograph fitted with an electron capture detector. The vaporized Otto Fuel II was 99% pure PGDN as measured by infrared analysis. 

Andren and Harris [54] have reported a methylmercury concentration of 0.02-0. lng Hg g 1 in unpolluted sediments by using a gas chromatograph with an electron capture detector. 

As mentioned previously, the scope of photochemical oxidants extends to organic nitrates and other carbonyl compounds. Among the organic nitrates, the one most often cited is peroxyacetylnitrate (PAN). Electron-capture detector techniques applied to the gas chromatt aph were used to measure PAN concentrations in Los Angeles late in 1%5. ... 

The use of a volatile solvent, e.g., pentane, was not explored because of inherent limitations. Concentration of such extracts was not possible because of the volatility of the sample components. Therefore the maximum concentration factor that could have been achieved was limited by the partition coefficients of the compounds into the solvent used in the extraction. For most compounds this factor was estimated to be about 10 1. Furthermore, with CRMS and other general detectors, the solvent masking problem would still preclude observation of many compounds. Therefore, the method would be limited to detectors that are not responsive to the solvent used in the extraction. Recent work (3.4,5) has indicated that extraction with a volatile solvent is a viable approach for the analysis of a small set of compounds, e.g., the trihalomethanes, with an electron capture detector in drinking water samples where concentration factors of 10 1 or less are acceptable. 

With enantiomer analysis, however, a linear detector response is indispensible. Thus, for the correct determination of. say, 0.1 % of an enantiomeric impurity, linearity within a concentration range of at least three orders of magnitude is required. It is generally accepted that the flame ionization detector (FID) does fulfill this requirement, but it is recommended that the linear detector response is verified via dilution experiments31. In contrast, the linear response range of the electron capture detector is low, being only two to three orders of magnitude. 

Some altitude effects on the operation of chromatographic instruments are anticipated. To achieve reproducible retention times for identifying compounds, mobile-phase flows need to be controlled so that they are independent of ambient pressure. Detectors may also respond to changes in pressure. For example, the electron capture detector is a concentration-sensitive sensor and exhibits diminished signal as the pressure decreases. Other detectors, such as the flame ionization detector, respond to the mass of the sample and are insensitive to altitude as long as the mass flow is controlled. 

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. 

DISCUSSION. Figure 10.11 shows the separation of a group of gases from blood. The procedure described depends on complete equilibration between the gas and liquid in the syringe. The period to achieve equilibration depends on the gas, and can be as little as 1 min for acetone and as much as 30 min for sulfur hexafluoride. Linear calibration curves can be obtained when peak height is plotted against gas concentration for either the flame ionization or electron capture detector. The reproducibility (n = 10) for ethane gave a standard deviation of 23% of the mean concentration while it was 2.3% for halothane and 1.8% for ether. 

Chlorofluorocarbon (CFC) compounds (Freons ) are important tracers of ocean circulation. Since CFC-11 and CFC-12 were released at different rates, their concentrations as well as their ratios can be used to determine when a water mass left contact with the sea surface over the 50 years since Freons were released into the atmosphere and taken up by the ocean. CFC-113 and carbon tetrachloride are being investigated as additional tracers. Picomole levels of CFCs dissolved in seawater are determined using a gas chromatograph equipped with an electron capture detector. The World Ocean Circulation Experiment (WOCE) Hydrographic Program calls for measurements with a precision and accuracy of 1% and a detection limit of... 

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