Physical and Analytical Aspects of a Microwave Excited Plasma

Abstract

A microwave generator operated at about 100 W was used to maintain a narrow plasma (6 cm long ×0.3 mm diam) in flowing argon at atmospheric pressure. A tapered rectangular cavity was used to couple microwave power to the plasma discharge. Approximate temperature and electron density measurements of the plasma were made with and without water being nebulized; the temperature measurements indicated a lack of thermodynamic equilibrium. The plasma discharge was examined as a source of excitation in atomic emission spectrometry. An aerosol (produced by a system consisting of a pneumatic sprayer, a heated chamber, and a special condenser) was introduced into a plasma having an excitation and ionization temperature of about 5000 K and an electron density of about 10¹⁵ cm⁻³ at a position 0.3 mm from the center of the plasma. Limits of detection were measured for cadmium, gallium, indium, mercury, and zinc. Appreciable signals were also observed for calcium, lead, strontium, and boron, but no limits of detection were measured.