Detektor wychwytu elektronów - podstawy teoretyczne i przykłady zastosowań

Abstract

A construction of the electron capture detector (ECD) and the physical phenomena of its operation are presented. On the basis of the experimental data and the developed physicomathematical model of the detector, the mechanism of the detector signal generation is explained. The paper contains description measurements of trace concentrations of chlorofluorocarbons (CFCs) and SF6 in air and water by the electron capture gas chromatographic method. The model derived allows to determine four different time constants related to the collection of electrons, the loss of electrons in the capture process by the impurity molecules, the loss of sample molecules by electron capture and the removing rate of molecules from the detector volume by the carrier gas. The electron capture efficiency coefficient, the detection and sensitivity coefficients of the detector have been defined. These coefficients are useful for estimation of the detector's detection limit. A method for preparation of the gas chromatographic measurement system with the ECD to trace analysis as well as ways of the determination of the detector optimal supply conditions in a constant frequency and a constant current modes of its operation are described. Examples of the ECD application in the long-term quantitative analysis of freons CCl3F (F-11) and CCl2FCClF2 (F113), chloroform (CHCl3), trichloroethane (CH3CCl3) and carbon tetrachloride (CCl4) in the atmosphere of Krakow in the period of 1997 to 1999 are presented. The monthly mean values of these concentrations have been compared with the data from Mace Head, the station situated at a similar latitude as Krakow. The gas chromatography measurements of freon F-11 in air were used to illustrate the influence of different interpolation methods on the accuracy and precision of measurements. It was shown that the 5-point Lagrange interpolation method yields the best accuracy. The reduction of the noise amplitude of a particular detector response leads to the increased signal to noise ratio and increases the accuracy of the measurements. The discrete wavelet transform (DWT) method for denoising of chromatograms has been used. It has been shown that the application of the DWT filtration leads to several times better measuring accuracy of the investigated compounds. The methodology of CFCs and SF6 analysis in water as anthropogenic tracers for hydrological applications is also presented. The extracted SF6 is measured by enrichment in a trap with glass pellets placed in liquid nitrogen and next desorbed to the gas chromatograph equipped with an ECD. The actual detection limit of SF6 is approximately 0.02 fmol/l.