PLASMA DIAGNOSTIC-TECHNIQUES FOR UNDERSTANDING AND CONTROL

To date, most success in overcoming plasma-based matrix interferences has been derived through empirical adjustment of operating conditions. However, further substantive gains are more likely to be realized by characterizing the interferences mechanistically and, through a more thorough understanding, to minimize them. In turn, achieving the necessary understanding is possible only through appropriate use of plasma diagnostic techniques. Moreover, such techniques can be useful not only to characterize, understand, and reduce matrix interferences, but also to monitor plasma and instrument operation. Using the diagnostic procedures to indicate instrument performance should eventually enable the entire instrument to be placed under computer control for continuous, rapid optimization in an on-line, adaptive fashion. In this paper, a number of important plasma diagnostic procedures are described and their se for characterizing interferences outlined. Included among the techniques are: (1) Thomson scattering for the determination of temporally and spatially resolved electron energy distributions and concentrations; (2) Rayleigh scattering for the temporally and spatially resolved measurement of gas-kinetic temperatures; and (3) Abel inversion and computed tomography for unravelling the true three-dimensional emission features of plasmas. Useful information about plasma operation can also be gained by the determination of noise power spectra, the acquisition of which is described. In addition, diagnostic techniques of particular use in plasma-source mass spectrometry are described. These include Langmuir-probe analysis for the characterization of localized space and floating potentials and ion-kinetic-energy determinations to enhance understanding of the expansion that occurs as a plasma is sampled into a mass spectrometer.