Analysis of aerosols via calibration-free laser-induced breakdown spectroscopy
Résumé
Large-scale industrial production of nanoparticles requires quality control through measurements of the size distribution and chemical composition. For this purpose, new on-line monitoring techniques allowing real-time and in-situ characterization have to be developed. Producing composite nanoparticles elaborated from several chemical elements requires an instantaneous and in-situ stoichiometry control. In this context, laser-induced breakdown spectroscopy (LIBS) is a promising technique. LIBS consists in focusing a powerful laser pulse on a material (solid, liquid, gas, nanoparticle flow) whose elemental composition is to be determined. The strong heating of the sample at the focusing spot leads to the ignition of a plasma. Simultaneous detection of all the elements is then achieved through optical emission spectroscopy. The LIBS technique has already been tested at INERIS for on-line monitoring, monitoring of heavy metal particle emission in the exhaust duct of a foundry and for controlling work environment. Thus, experiments aiming at determining the stoichiometry of aerosols using a calibration-free LIBS technique have been carried out. The experimental setup consists of an analysis cell coupled to a LIBS optical setup. A vortex mixer has been used to suspend alumina Al2O3 micrometric particles. The suspended particles are then flowed through the cell using an inert gas (argon/helium). Laser pulses of 5 ns duration originating from a Q-switched Nd:Yag laser (1064nm) were focused inside the analysis cell on the aerosol flux where the plasma was generated. The LIBS signal was collected from the plasma using a telescope and spectra containing lines of aluminium Al and oxygen O were recorded using an echelle spectrometer coupled with an intensified charge-coupled device detector. The recorded spectra were then compared to theoretical spectra calculated for a plasma in the Local Thermodynamic Equilibrium . We demonstrate that the stoichiometric composition of the alumina particles can be deduced from the best agreement between measured and computed spectra if the experimental conditions were properly chosen. These preliminary experiments show that stoichiometry of alumina particles may be determined on-line without resorting to calibration with standards.