Accurate and sensitive metals emissions monitoring with an atmospheric microwave-plasma having a real-time span calibration

Both high sensitivity and good measurement accuracy are required in a metal s continuous emissions monitor in order for it to be acceptable for complia nce monitoring. An atmospheric plasma sustained by microwaves with an attac hed source of a calibrated trace metals aerosol has been shown to be capabl e of achieving both of these requirements. The microwave plasma is continuo us and operates in undiluted stack exhaust for atomic emission spectroscopy of trace metals. The plasma is sustained in a shorted waveguide that is at tached to the stack by a short sample line (< 50 cm). It is powered at 1.5 kW, at a frequency of 2.45 GHz. An undiluted stack slipstream is isokinetic ally drawn into the plasma by a suction pump at a nominal flow of 14 l/min. The pneumatic nebulizer attached to the sample line can momentarily, on co mmand, inject a known concentration of metals solution providing a real-tim e span calibration. The system was tested on the exhaust stack of the rotar y kiln incinerator simulator facility at the Environmental Protection Agenc y (EPA) National Risk Management Laboratory in Research Triangle Park. Thre e hazardous metals were monitored, lead, chromium, and beryllium. These mea surements were referenced to EPA Method-29. A total of 20 spiked stack exha ust tests were carried out. Ten one-hour tests at high concentration (40-60 mu g/actual m(3)) and ten one and half-hour tests at low concentration (10 -15 mu g/actual m(3)). The microwave plasma monitor achieved relative accur acies of approximately 20% for lead and beryllium and 40% for chromium with a threshold detection capability of < 3 mu g/actual m3 for a time response of similar to 1 min. The relative accuracy deviation from the EPA Method-2 9 was found to be mostly systematic. This suggests the possibility of using a site-specific calibration to bring the microwave plasma into compliance with EPA's goal of 20% relative accuracy to the reference method. Laborator y work is continuing to add mercury, arsenic, and cadmium to the monitored metals. Mercury and arsenic present a particular challenge to achieving hig h detection sensitivity in undiluted stack exhaust because the plasma is le ss efficient in exciting Hg and because UV absorption interferes with As de tection. This is a problem that is also in common with air ICP and in situ spark plasma methods for continuous emissions monitoring of metals. (C) 200 0 Elsevier Science Ltd. All rights reserved.