Studies of a helium-operated gas-sampling Grimm-type glow discharge for the atomic emission spectrometric determination of chlorine in halogenated hydrocarbon vapors

Abstract

Chlorine originating from halogenated hydrocarbons introduced through a fused-silica capillary was detected by atomic emission spectrometry from a helium-operated gas-sampling Grimm-type glow discharge. During the introduction of the halogenated hydrocarbon vapor, the operating voltage of the discharge was found to increase considerably. The device showed excellent discharge and signal stability over periods of several hours. It was found that the noise characteristics of the discharge were virtually white and that they did not change upon introduction of dichloromethane. Spatially resolved measurements performed with an automated x– y translation stage and end-on observation demonstrated that the emission-intensity distribution of both chlorine and helium is dependent on the geometry of the entrance aperture in the cathode block. For conical and hollow-cathode-like entrance arrangements, emission intensities were concentrated at the entrance of the capillary. However, for a flat cathode sampling plate, the intensity profiles exhibited a minimum at the entrance orifice. Background intensities were found to change during the introduction of halogenated hydrocarbon vapors, necessitating both careful background acquisition and correction. After discharge conditions were optimized (at 25 mA and 7 Torr), the detection limit for chlorine utilizing the Cl II 479.5 nm line was found to be 20 ng s −1. Vapor from aliphatic as well as aromatic hydrocarbons were introduced into the discharge. To a first approximation, signals were found to be proportional to the amount of both chlorine and carbon released into the discharge, enabling the ratio of carbon to chlorine to be determined. The device therefore deserves further study regarding its potential as an element-specific detector for gas chromatography.