Liquid electrode dielectric barrier discharge for the analysis of solved metals

Abstract

A miniaturised dielectric barrier discharge at atmospheric pressure is used for the detection of metals dissolved in liquids by means of optical emission spectrometry. The plasma is formed inside a fused silica capillary with an inner diameter of 700 μm between the flowing liquid surface and a tungsten electrode with 400 μm diameter. In this case, the flowing liquid with 1 mol L⁻¹nitric acid forms the other electrode of the plasma. A liquid flow rate of 20 μL min⁻¹ is used and the pulsed high voltage with 3.2 kV_pp and 87 kHz is dielectric coupled into the liquid over a copper coating on the outer capillary surface, where the dielectric barrier is formed by the wall of the capillary. Disequilibrium between the liquid flow rate and transfer rate into the plasma causes a pulsation of the discharge. This pulsation shows a positive effect on the long-term stability of the discharge, where the discharge can be sustained in the range of one hour and more. The stability of the discharge is influenced also by the nitric acid concentration and therefore the conductivity of the liquid. Calibration measurements are performed for several alkali, alkaline earth metals and silver. Detection limits are determined between 0.02 mg L⁻¹ for potassium and 6.9 mg L⁻¹ for barium. Together with the achieved stability and low flow rate the application as a monitoring device in flow systems or coupling with micro-separation devices seems possible.