Effect of oxygen concentration on the detection of mercury in an atmospheric microwave discharge

A microwave plasma combined with atomic emission spectroscopy is being developed for real time emission monitoring of metals in smoke stacks. The plasma is sustained by 1.5 kW,2.45 GHz microwave radiation in a gas flow at atmospheric pressure. The addition of oxygen to a nitrogen gas stream reduces dramatically the emission intensity of the 253.65 nm mercury transition, while at the same time it increases the emission intensity of the 405.78 lead line and other metal transitions. Measurements asa function of oxygen concentration in the plasma gas stream on the intensity of the 253.65 nm mercury line and on the excitation temperature, T-exc, are presented. Possible mechanisms in the plasma that can effect the emission strength of the Hg atom are explored. Plasma absorption at 253.65 nm by ozone is ruled out by UV transmission measurements. The measurements suggest that a slight cooling of T-exc with the addition of oxygen, combined with the sharp electron impact excitation cross section can explain the observed behavior.