Appearance of ICP plasma:

The typical plasma has an extremely intense and brilliant white, nontransparent core topped by a flame like tail as shown in Figure. The analytical zone has a temperature of 5000-8000 K and is approximately 1 cm above the coils. It offers the best optical viewing area for maximum sensitivity. Figure shows the variation of temperature throughout the plasma. Plasma emits a continuum of background radiation that extends from the visible into the ultraviolet region. The radiation originates from electrons, Ar and Ar⁺ ions, as well as various atomic and molecular species in the matrix and their recombination products. Within the region 10 to 30 mm above the core, a continuum fades, and the plasma is optically transparent. The spectral observations are generally made at a height of 15 to 20 mm above the induction coil. There the background radiation is remarkably free of argon lines and is well suitable for analysis.

Figure: A schematic diagram showing the temperature profile of a typical ICP torch