Microwave Plasma Continuous Emissions Monitor (MP-CEM)

[摘要] A compact microwave plasma device that can be attached to a stack for near in situ atomic emission spectroscopy, with a real-time span calibration injector for accuracy, is under development for continuous metals monitoring. It uses a commercially available 2.45 GHz, 1.5 kW microwave source to generate a plasma in an undiluted, isokinetic sustained in a shorted waveguide, which together with a short sample line is maintained at high temperature (>150 C) to avoid condensation. Relative to other plasma based CEMs the microwave plasma has a significant advantage to continuously operate reliably in large volumes (- 50 cm3) of fast flowing (> 14 1/minute, up to 40 ft/sec) air or undiluted stack exhaust. A pneumatic nebulizer attached to the sample line can momentarily, on command, inject a known trace concentration of metals to provide a real-time span calibration whenever needed. Novel grating spectrometers using low cost detectors have been used for simultaneous multiple metals monitoring with continuous rapid signal acquisition making possible the observation of discreet particles. Since the particles are observed as plasma induced sources of light, the usual limits on monitoring discreet submicron particles by conventional external light instruments are not encountered. Detection limits for Be, Cr, and Pb of < 3 ug/m3 have been shown for one minute signal averaging in plasmas with 6% water content. A one minute detection limit for Hg of < 1 ug.m3 has been recently achieved in ambient laboratory air plasmas using a dedicated spectrometer. Lower detection limits are possible with longer signal integration time. Pasting testing on an EPA research incinerator demonstrated the capability for monitoring metal concentrations with a relative accuracy of 20% to EPA method 29.

[发布日期] 2000-12-01 [发布机构] Massachusetts Institute of Technology (United States)

[关键词] Calibration;Incinerators;Emission Spectroscopy;Air Pollution Monitors;Microwave Radiation