Radial excitation temperatures in argon-oxygen and argon-air inductively coupled plasmas

Abstract

Abel-inverted excitation temperatures are measured for Ar-O₂ and Ar-air inductively coupled plasmas (ICPs) formed in a conventional torch with a 27.12-MHz crystal-controlled generator. The dependencies of temperature on gas composition (0, 5, 10, 20, 50 and 100% of either O₂ or air in the outer flow), forward power (1.2 and 2.5 kW), observation height (6, 10 and 15 mm) and injector gas flow-rate (1 and 2 l min^–1) are studied using Fe and Mo as the thermometric species. Results are compared with temperatures measured for Ar and Ar-N₂ plasmas. When 5–100% O₂ or 5–100% air is used in the outer flow instead of pure Ar, the axial temperatures measured at the 10-mm observation height and 1.2-kW forward power are greater than that of the commonly used Ar ICP, except when pure O₂ is used in the outer flow. The highest axial temperatures are achieved when only 10% O₂(6800 K) or 10% air (6400 K) are introduced into the outer flow. At 2.5 kW and 6 mm, the Ar-air plasma, with air in the outer flow, exhibits a temperature of approximately 6200 K, similar to that of a 2.5-kW Ar-N₂ ICP or a 1.2-kW Ar ICP. In contrast, the axial temperature measured for a 2.5-kW Ar-O₂ ICP with pure O₂ in the outer flow is approximately 1500 K lower than that of the commonly used 1.2-kW Ar ICP. In general, the trends noted for the Ar-air ICPs are similar to those described earlier for the Ar-N₂ ICPs.