DESTRUCTION OF HALOGENATED METHANES BY NON-ELECTRON CAPTURE PROCESSESWITHIN AN ELECTRON-CAPTURE DETECTOR

The rates of destruction of 8 halogenated methanes by processes other than dissociative electron capture (EC) reactions have been measured w ithin an ionization cell for which conditions are identical to those e xisting within an electron capture detector (ECD) for gas chromatograp hy (GC). It is shown that additional destructive processes are distinc tly operative for CCl4 and CBrCl3, but not for CH2Br2, CH3I, CHBr3, CH FBr2, CF2Br2, and CHCl3. In these measurements, CFCl3 serves as a refe rence compound for which no extra-EC destruction with an ECD is assume d. This assumption is based on previous uses of CFCl3 as a reference c ompound in related studies and on additional measurements provided in the present study. It is shown that the most likely mechanism for the extra-EC destruction of CCl4 is a reaction of this compound on the wal ls of the ion source which have been chemically activated by the depos ition of gas phase free radicals. The rate of this wall-assisted destr uction of CCl4 is increased with use of lower temperatures and approac hes unit efficiency upon wall collisions at room temperature. The impl ications of these results for the quantitative analysis of halogenated compounds by the ECD are discussed.