Size-resolved analysis of fine and ultrafine particulate matter by laser-induced breakdown spectroscopy

Abstract

In emissions originating from industrial processes, elemental concentration characteristically depends on particle size. Therefore, not only is the average chemical composition of the particles in the focus of interest but also their size-dependent composition. For industrial applications such as on-line process control, a system using laser-induced breakdown spectroscopy (LIBS) is developed providing the analysis of particulate matter directly in an air stream with a short response time to changes in elemental composition. By using a co-flow in addition to the aersosol flow the time resolution of the system can be enhanced. Investigations are undertaken concerning properties of the laser plasma, the analysis of monodisperse and polydisperse aerosols with regard to the relative elemental concentration and the mass concentration of the particles. The system can be calibrated to either total elemental concentrations or relative compositions. The signal response to a variation of elemental concentration as well as to the particle mass concentration show kinds of saturation effects. Temporal plasma evolution for CaCl₂ particles and pure argon are investigated. Different decay times for argon and the particle matter depending on the origin of lines within the plasma are observed. Electron density and temperature are determined using Ca emission lines to describe the temporal evolution of the plasma state for the particle analysis. It has been observed that the electron density increases rapidly for plasma temperatures above 10 000 K. Size-resolved analyses of particles in the size range from 20 nm up to 800 nm are carried out. It has been found that the signal response correlates with the mass distribution of the particles.