Group 5 147 Electron capture

In this group the most commonly used reaction is that of radiative neutron capture. Also here are to be found (n, 2 n) and (y, n) reactions, although very few studies have been done with these reactions, and isomeric transitions (although these may often be more profitably discussed along with electron capture reactions). 

The presence of heteroatoms usually provides a convenient feature for improving selectivity by employing selective detection mechanisms. GC may then use flame photometric detection (FPD) for S and P atoms and to a certain extent for N, Se, Si etc. thermoselective detection (TSD) and nitrogen-phosphorus detection (NPD) for N and P atoms electron capture detection (ECD) for halogen atoms (E, Cl, Br, and 1) and for systems with conjugated double bonds and electron-drawing groups or atomic emission detection (AED) for many heteroatoms. 

To improve the detectability of silyl ethers, silylation reagents containing an electron-capturing group [443,449-451,468] or cyano group for thermionic detection [469] have been prepared, the 2-cyanoethyldimethylsilyl derivatives are only marginally aore sensitive (ca. 5 fold) to the thermionic detector than to the flame ionization det ftpr which Units their usefulness. The... 

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... 

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

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,... 

Radioisotopes are unstable and decay by particle emission, electron capture, or y-ray emission. The decay is a random process, i.e., one cannot predict which atom from a group of atoms will decay at a specific time. The decay of radioisotopes, therefore, is described in terms of the average number of radioisotopes disintegrating during a period of time. The disintegration rate (or the number of disintegrations per unit time), -dN/dt, of a radioisotope at any time is proportional to the total number of undecomposed radioisotopes present at that time. This may be expressed as follows ... 

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