Close-up of three microwave plasma sources in view of improved element-specific detection in liquid chromatography

Abstract

Progress in the features of three types of microwave plasmas is discussed, in view of the development of successful methods for atomic spectrometric element-specific detection in liquid chromatography. For the low-power microwave induced plasmas the development of the toroidal plasma in a TM010 cavity according to Beenakker is mentioned as the break-through for the introduction of wet aerosols. Capacitively coupled microwave plasmas (CMP), which can be operated with helium, argon and even air as working gases, are robust and allow obtaining of detection limits for Fe, Cr, Ni and Co in aqueous solutions in the 0.02 to 0.06 μg/ml range and in light oils, as an example of organic liquids, between 0.08 and 0.13 μg/ml. Special attention should be given to the microwave plasma torch (MPT) in which aerosols from aqueous as well as from organic solutions produced by a Legere nebulizer can be introduced without desolvation. Here, detection limits for Cd, Cr, Li and Pb range from 0.02 to 0.5 μg/ml. For the case of chromium dissolved as dithiocarbamate complex in an acetonitrile/H2O mixture (2:1), its detection limit is 0.12 μg/ml, being already below that obtained with UV spectrophotometry. The limits of detection achieved with the sources discussed in the case of atomic emission spectrometry show the prospective for further development.