A detailed study of the importance of various spatial dimensions in an ICP torch is described. Of the various dimensions which were examined, the annular spacing between the coolant (outer) and plasma (flared) tube is the most critical for plasma stabilization at low argon coolant flows and applied radio-frequency powers. Similarly, the inner diameter of the aerosol injection tube was found to be important, with the value of 1.0 mm chosen for a compromise between low-power, low-flow capability and routine analysis of high-solids sample solutions. A constriction at the inlet of the coolant argon was found not to affect plasma stability greatly, but altered significantly the applied powers and coolant flows which were necessary for plasma ignition. With these and other torch dimensions optimized, it was found to be possible to ignite a plasma at a coolant flow of 5 L/min and an applied radio frequency power of 450 W. Continued operation of the plasma during analysis of real samples or during aspiration of a 1% NaCl solution was found to be possible alternatively at 500 W (rf power) and 3.5 L/min coolant argon or 125 W and 5.5 L/min, respectively.
Modular ON/OFF and Phase-Shifting for High-Speed Radio Frequency Power Modulation