An emission-controlled feedback system for stabilizing an ICP has been investigated. Unlike previous stabilization methods which monitor only the power being sent to the plasma, the present one uses an emission signal from either argon or an internal reference element as a stability indicator. Using a previously constructed power-modulation circuit for rf power control, the feedback network adjusts the plasma power to compensate for changes in the emission level of the reference element. It is shown to be particularly useful for reducing or eliminating the drift that occurs when the plasma is warming up. A key point for successful operation of the feedback arrangement is that both the feedback-signal and the analysis photometric systems must receive light from exactly the same region of the plasma. Also, the excitation energy and ionization characteristics of the reference element should be comparable to those of the desired analyte. Results with several combinations of argon, barium, calcium, iron, manganese, sodium, and strontium lines are presented.
M. A. Marks and G. M. Hieftje, "An Emission-Based Feedback System for Stabilizing an Inductively Coupled Plasma," Appl. Spectrosc. 42, 277-280 (1988)