PERFORMANCE OF A MICROWAVE-INDUCED PLASMA (MIP) OPERATED IN A LIQUID-COOLED DISCHARGE-TUBE FOR ATOMIC-EMISSION SPECTROMETRY

Different types of microwave induced plasma (MIP) discharge operated i n liquid-cooled tubes, namely a glass tube of Duran(R), a quartz tube of Herasil(R), and a very simple demountable discharge tube made of gl ass and quartz have been investigated. The last tube leads to the best analytical properties and the longest lifetime. The intensities of si licon lines and of the continuum spectral background, together with th e signal-to-background ratios for B, Ca, Cd, Co and Zn in the case of the pneumatic nebulization of solutions have been measured and used as an indicator for the cooling efficiency. The MIP torch was cooled wit h a thermostated silicon oil, The decrease of the temperature of the c ooling medium causes a measurable decrease of the spectral background intensity. Diagnostic measurements of the plasma include radial profil es of spectral line intensities and excitation temperatures with the l ines of Fe I; values of 5000-6000 K are found. The influence of differ ent plasma parameters, e.g. microwave power and helium flow rate, is i nvestigated. The preliminary analytical characterization of a helium M IP maintained with the liquid-cooled demountable discharge tube is pre sented. Limits of detection for Al, B, Ca, Co, Fe, P, Sb and Zn (betwe en 0.002 and 1.2 mu g ml(-1)) are comparable with or better than those reported for low power helium MIPs with sample introduction in the fo rm of a wet aerosol.