Microwave-induced plasma as an element-specific detector for speciation studies at the trace level. Invited lecture

Abstract

The microwave-induced plasma (MIP) is a promising excitation source for atomic spectrometry. In 1976 a major breakthrough was made by Beenakker, who introduced a cylindrical TM₀₁₀ cavity capable of operating at atmospheric pressure for argon and helium plasmas. The helium-sustained MIP is particularly attractive because both non-metals and metals can be detected with good sensitivity. The characteristics of the MIP as an atomization and excitation source are briefly discussed and some advantages (multi-element detection of non-metals and metals) and limitations (only small amounts of sample can be tolerated) are highlighted. The MIP is efficient in the excitation of elements but the limited thermal energy available leads to difficulties with sample desolvation and atomization. In order to overcome the problems several techniques have been investigated to introduce volatilized samples into the MIP. Coupling with gas chromatography (GC), electrothermal vaporization, chemical vapour generation and other techniques is also discussed. The primary focus of work in this area has dealt with the utilization of the MIP as an element-specific detector for GC. With MIP atomic emission spectrometry it is possible to investigate chromatographic processes and to determine empirical molecular formulae of materials eluted from a GC column. Combination of the MIP with separation techniques for speciation of organic and organometallic compounds even at trace concentrations is the most important application for many disciplines, such as medicine and toxicology. Some of the useful features of MIP detection are exemplified by the determination of specific organomercury compounds at trace levels in environmental samples.