Electron-capture coefficient

The electron capture coefficient is the experimental molar response of the ECD normalized to a constant concentration of electrons. This concentration may change because of changes in impurities in the carrier gas. It is best to minimize the changes, but if it is assumed that the reactions of the impurities only reduce the concentration of electrons and do not react with the specific test molecule, the correction is simply a multiplication factor. The electron capture coefficient Khcd is calculated from the concentrations of the electrons //, in the absence of the test molecule, the concentration of the electrons in the presence of the test... 

By assuming that the electron capture coefficient is equal to the equilibrium constant for the reaction of thermal electrons with aromatic hydrocarbons, the electron affinity can be obtained by measuring the response as a function of temperature. The equilibrium constant Keq is related to the electron affinity of the molecule by... 

The electron capture coefficient for nondissociative electron attachment is... 

Wang and Charles Han calculated the electron affinities of aldehydes and ketones by using the parameterized Huckel theory. Eight parameters were used to calculate the electron affinities of 16 compounds with a deviation of only 0.05 eV. However, some of the data were not published until the 1970s [35]. By measuring relative electron capture coefficients and scaling to the acetophenone data, more precise electron affinities could be obtained. This was further support for the validity of the ECD model. M. J. S. Dewar reproduced the experimental electron affinities of aromatic hydrocarbons using the MINDO/3 method and calculated Ea from reduction potentials [36]. 

The greatest support for the actual mechanistic steps rests in the concentration dependence of the response (Equation 1) and more importantly the temperature dependence of the electron capture coefficient. The temperature dependence of the capture coefficient, as it pertains to the various mechanisms associated with Reactions I-IV, are discussed below, and agreement with experiment is strong supporting evidence for the kinetic model. The interpretation of the temperature dependence in terms of molecular parameters, such as molecular electron affinities or... 

For Eq. (3.10) and Eq. (3.12) the bimolecular rate constants can be replaced with pseudo unimolecular rate constants within the limits of either fractional electron capture or constant positive ion concentration. All the above reactions take place on a time scale that is fast relative to the time required for transport through the detector. Under steady state conditions the electron capture coefficient K (see Eq. 3.7) is given by... 

For compounds that capture electrons predominantly by a single reaction pathway some rate constants are negligible and Eq. (3.13) can be simplified. The model can be extended to include stable excited states that are involved in the electron-capture process for a minority of compounds [315]. The detector response is also a function of the cleanliness of the detector represented by the contribution of ki[I] to the electron capture coefficient. 

In Equation 6.16, Is is the standing current, I the current measured when electronabsorbing species are in the detector at concentration c, and A is a constant characteristic to the cell and the species present (also known as the electron-capture coefficient). 

ECD Electron capture detection Ko, Kqc Soil sorption coefficients... 

The value of tt is evaluated [177] as xt (2C i ), where C is the coefficient of electron capture by a trap is the local density of electrons at the instant the Fermi level intersects the trap level when a direct bias signal is applied to the barrier. 

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. 

We denote the capture coefficient for electron traps by /3 and that the recombination by 7. One can obtain... 

Chemical evidence for the existence of electrons in irradiated cyclohexane was obtained from pulse radiolysis studies of solutions containing aromatic solutes27. Because of the lifetime of the pulse these experiments only allowed the determination of ions still surviving after 10 6 sec. With benzophenone and anthracene as scavengers transient absorption peaks at 700 nm and 730 nm respectively, were obtained. These were consistent with the known spectra of the benzophenone and anthracene radical ions and are most simply accounted for by assuming direct electron capture by these solutes. Positively charged ion radicals may also be produced since these are likely to have similar spectra. Ion yields can be calculated since the absorption coefficients are known, but these yields necessarily represent the sum of the positive and negative ion yields. Some results are shown in Fig. 3. 

Figure 7 compares the results obtained from the average of four subjects analyzed by GLC-MS-EI, TLC, radio-label, and electron capture GLC. Correlation coefficients are calculated in Figure 8. The results are in reasonable agreement, and in particular the GLC-MS and electron capture GLC procedures gave good agreement for most points over the whole curve.

Fig. 18.4. The excitation photon emissivity coefficient for C II (2s23s2S-2s22p2P) at 858 A, driven from the ground term 2s22p2P. The emissivities are calculated in the GCR picture and so there are equivalent entries in the database for emissivities driven by excitation from the metastable 2s2p2 4P and by recombination (free electron capture) from 2s2 4S and 2s2p3P and by charge transfer - also from the latter two parent terms...

The external standard was PCA-60, except as noted. LRMS = low resolution mass spectrometry HRMS = high resolution mass spectrometry ECD = electron capture detector SIM = selected ion monitoring TIC = total ion current CV = coefficient of variation ADM = average deviation from mean nd = not determined. b True concentration (by weight) = 74 ng/pl. c True concentration (by weight) =118 ng/pl. 

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