Aerosols analysis via calibration-free laser-induced breakdown spectroscopy in helium
Résumé
Particle detection and characterization are new challenges accompanying the nanotechnology industry emergence as well as environmental survey. Recently, particular efforts have been dedicated to the development of in situ real-time measurements of size and composition of particles in aerosols or liquid suspensions. Laser-induced breakdown spectroscopy (LIBS) is a promising technique for such type of analyses. However, the variability of particle size and density make the measurement calibration difficult. To overcome this problem, we have developed a calibration-free LIBS measurement procedure. Based on modeling of the laser-induced plasma, the composition of aerosols is deduced by comparing the measured emission spectrum to a spectrum computed for plasma in local thermal equilibrium (LTE). To illustrate the feasibility, the breakdown was produced in helium at atmospheric pressure. Due to the high initial temperature and the large thermal conductivity, breakdown in helium leads to rapid aerosol vaporization and more uniform plasma, compared to breakdown in air or argon. We demonstrate that the plasma can be described by a partial thermodynamic equilibrium. Thus, the population number densities of all plasma species follow the Boltzmann equilibrium distributions, except of helium atomic and ionic number densities. By recording spectra at different delays with respect to the laser pulse, we show that accurate analyses of the aerosol composition are only possible for t≤1μs, when the electron density is large enough to ensure the collisional equilibrium for the aerosol species. The present results agree with recent CFLIBS analysis of solid alumina and glass, and encourage aerosols compositional analysis using calibration-free laser-induced breakdown spectroscopy.