Abstract

Atomic emission spectroscopy of a continuous, atmospheric-pressure microwave sustained plasma in an undiluted slip stream of stack exhaust is under development for real-time monitoring of EPA-regulated hazardous metals (As, Be, Cd, Cr, Hg, and Pb). The detection limit in such a plasma for two of these metals at their strongest transitions, Cd I 228.8 nm and Hg I 253.65 nm, has been found to depend on the path length between the plasma and the detection system. Measurements were carried out on long plasma columns (>100 mm × ∼10 mm diameter) sustained in air by approximately 1.5 kW power at 2.45 GHz. The detection limits, using a long path-length axial view for UV light collection versus short path-length side views at different points along the plasma column, have shown a clear diminution of the signal for the axial view. Self-absorption by unexcited atomic cadmium and mercury is shown to be responsible for diminution of the axial signal. The Cd I 228.8 nm transition was observed to reverse from an emission feature to an absorption feature when the axial path-length for the light was extended.