Aerosols analysis by LIBS for monitoring of air pollution by industrial sources
Résumé
In the context of the air quality improvement, there is an increasing need to monitor gas and particle emissions originating from exhaust stacks (incinerators, foundries, etc.) for regulation enforcement purposes. Lots of pollutants are targeted; among them, heavy metals are mostly found in particulate forms. Hence, there is a need to promote the development of suitable on line analytical techniques. To that end, laser-induced breakdown spectroscopy (LIBS) appears to be a good technique. Indeed, it is quantitative, fast (<1 min), requires no sample preparation, and can be performed at remote distance. The instrumentation is compact and offers the possibility to be used for continuous and in-situ monitoring. Two different approaches have been tested by several authors to analyze aerosols by LIBS, by focusing the laser either on particles collected on a filter or directly into the aerosol. In this work, these two approaches, aiming at measuring the mass concentration of micrometer metallic particles in air, are investigated and compared. The experimental setup includes an aerosol source (an ultrasonic nebulizer producing a diluted aerosol of CuSO4 particles) ; two sampling lines for particle sizes and, for reference concentration measurements, a line for direct LIBS analysis ; and a fourth one devoted to filter sampling for subsequent LIBS measurements. Calibration curves were obtained with those two experimental approaches and the results are compared. In terms of sampling particles number, indirect analysis appears to be more efficient than direct analysis for our experimental conditions. Better detection limits were found with direct analysis when comparing the two approaches under similar sampling conditions (analysis time and sampling flow).