The spatial dependence of emission from a microwave-induced plasma in argon has been studied. A graphite furnace atomizer was used as a means of sample introduction. Emission from metallic elements is localized to a few cm near the inlet of the discharge, but the exact position of the emission profile is element-dependent. For non-metals, such as I, a broad profile centered about the resonant cavity is found. This difference in behavior is shown to be due to the deposition of metals upon the walls of the discharge tube used to confine the plasma. The removal of analyte atoms is explained by a mechanism which involves ionization of the analyte and then radial acceleration of these ions under the influence of the inhomogeneous microwave field. This hypothesis accounts for the observed decrease in emission intensity as microwave power is increased.
J. P. Matousek, B. J. Orr, and M. Selby, "Spatial Emission Properties of a Microwave-Induced Plasma," Appl. Spectrosc. 38, 231-239 (1984)