DEVELOPMENT OF A JET-REMPI BASED CONTINUOUS EMISSIONS MONITOR FOR DIOXINS AND FURANS

摘要

The need for a continuous emissions monitor for the determination of dioxins and furans at realistic (ppt or sub-ppt) concentrations in real time (minutes) is widely recognized in the waste combustion community. Such an instrument would enable rapid experimentation on incinerators, so as to test and develop control strategies. It would also be extremely useful in laboratory kinetics experiments seeking to understand formation, interconversion, and destruction processes for dioxins and their precursors. The key issues are overall sensitivity, and selectivity among the many congeners found in real applications. At SRI International we are developing an instrument to meet these needs. Our instrument is based on supersonic jet expansion and cooling, followed by resonantly enhanced multiphoton ionization (REMPI) into a mass spectrometer. This furnishes the dual selectivity of tuned laser absorption and mass analysis. We have examined a number of mono- and dichlorinated dioxins, including mixtures of similar congeners in the 50-100 ppt range. Using the well known spectroscopy of benzene, we have characterized the internal temperature in our gas expansion and investigated its variation with distance from the nozzle exit, as well as the corresponding gas density distribution. This characterization will lead to improved sensitivity through optimization of the ionization conditions.