Observation of self absorption of mercury I and cadmium I emission in an atmospheric microwave sustained plasma

Atomic emission spectroscopy of a continuous, atmospheric-pressure microwav e sustained plasma in an undiluted slip stream of stack exhaust is under de velopment for real-time monitoring of EPA-regulated hazardous metals (As, B e, Cd, Cr, Hg, and Pb). The detection limit in such a plasma for two of the se 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 det ection system. Measurements were carried out on long plasma columns (>100 m m x similar to 10 mm diameter) sustained in air by approximately 1.5 kW pow er at 2.45 GHz. The detection limits, using a long path-length axial view f or UV light collection versus short path-length side views at different poi nts along the plasma column, have shown a clear diminution of the signal fo r the axial view. Self-absorption by unexcited atomic cadmium and mercury i s 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 abso rption feature when the axial path-length for the light was extended.