The Use of Pulse-Interrupted Radiofrequency Excitation to Study Energy Transport and Analyte Excitation in the Inductively Coupled Plasma

Abstract

Extensive study of the analytical inductively coupled plasma (ICP) by conventional spectroscopic techniques has failed to produce an adequate model of the discharge. That such is the case was emphasized in a recent summary by L. de Galan of efforts to model the ICP. The discharge is not in local thermodynamic equilibrium. An understanding of the microscopic processes that give the plasma its desirable analytical characteristics is difficult to obtain because conventional measurement techniques record only steady-state values in a dynamic system. These values reflect a balance among a variety of competing or interactive processes, without providing specific information about what those processes may be. Some information about energy transport and excitation mechanisms is present in the spatial structure of the ICP. Any model of the ICP must account for observed spatial variations in emission intensities, particle densities, and temperatures. Additional information is present in the spatial and temporal changes in the same quantities as the power applied to the plasma is systematically varied. Hieftje et al. have described experiments in which they sinusoidally modulated the power applied to an ICP, then monitored phase and magnitude changes in signals from the plasma as the frequency was varied. Interpretation of these experiments has been difficult because the response of the plasma is extremely nonlinear, and because rate information is present only indirectly in the form of frequency-dependent phase shifts and rolloffs in the magnitude of signals from the plasma. This note describes preliminary experiments with an alternative form of power modulation that promises to yield more direct results. The power applied to an analytical ICP was repetitively pulsed off, and emission signals were monitored as a function of position and time.