Selective detection of iodinated hydrocarbons by the electron capture detector with negative ion hydration and photodetachment

Abstract

An electron capture detector (ECD) is described that responds sensitively to trace levels of iodinated hydrocarbons but does not respond to chlorinated hydrocarbons (ClHCs). These response characteristics have been achieved through control of four gas-phase processes: dissociative electron capture (EC) by the halogenated hydrocarbons, the regeneration of an EC-active species (thought to be HI) by the recombination of I{sup {minus}} with positive ions, the photodetachment (PD) of I{sup {minus}}, and the prevention of PD of Cl{sup {minus}} by the clustering of this negative ion by water molecules. This photodetachment-modulated electron capture detector (PDM-ECD) is instrumentally simpler than an earlier version. Its light source is a low-power Hg arc lamp and requires no associated monochromator or light filters. The application of this instrument to the specific detection of methyl and ethyl iodide in an air sample that contains an excess of ClHCs is demonstrated. The device is also shown to hold considerable promise for the selective detection of brominated hydrocarbons in the presence of ClHCs.