A COMPARATIVE-STUDY OF 3 MICROWAVE-INDUCED PLASMA SOURCES FOR ATOMIC-EMISSION SPECTROMETRY .1. EXCITATION OF MERCURY AND ITS DETERMINATION AFTER ONLINE CONTINUOUS COLD VAPOR GENERATION

Argon and helium microwave induced plasmas (MIPs), sustained in a Beenakker cavity (with ''capillary tube'' and ''tangential flow'' torches), a surfatron and a microwave plasma torch (MPT) have been compared in terms of their discharge properties (plasma configuration, stability and gas consumption) and ability to excite Hg atoms. An on-line continuous mercury cold vapour generation system, using SnCl2/HCl as the chemical reducing agent, was employed as the sample introduction system. Analytical figures of merit for the determination of mercury by atomic emission spectrometry (AES) showed the superiority of He discharges over the argon plasmas as excitation sources of atomic mercury. The He surfatron, with a 1 mm i.d. fused silica tube as the plasma torch, turned out to be the cavity that offered the best Hg(II) detection limit (10 pg ml-1), a linear dynamic range of more than three orders of magnitude, and a precision of +/-4%. However, the plasma generated in the surfatron device used was shown to be susceptible to the entrance of molecular gases, e.g. produced during the sample reduction step, and to water vapour. Application of these systems to the sensitive determination of mercury in sea-water has been explored.