Continuous monitoring of toxic metals in gas flows using direct-current plasma excited atomic absorption spectroscopy

A measurement apparatus employing direct current (dc) plasma excited atomic absorption spectroscopy was developed and demonstrated for continuous meas urement of toxic metals in process gases. Process gas is continuously sampl ed along a heated sample line. Metal compounds contained in the gas are the rmally decomposed by mixing the gas with a plasma jet produced with a dc ni trogen plasma torch. Transmission of monochromatic light is measured throug h the gas jet, and absorbance caused by metal atoms is distinguished from t he background by means of the Zeeman effect. The metal concentration in the sample gas is calculated from the measured absorbance with the known dilut ion and decomposition factors taken into account. The detection limits of t he current prototype are 0.04 mg/m(3) for cadmium and 0.4 mg/m(3) for lead. The measurement accuracy is better than 20%, and the maximum measurement r ate is about 100 values per minute. The instrument was designed to withstan d wet, corrosive, and particulate-laden flue gases at temperatures up to 11 00 degreesC. The instrument can also be used, after minor modification, for measurements at pressurized conditions. The performance of the instrument was demonstrated in connection with a real fluidized bed combustor.