The quality control of nanoparticles in industrial production requires on-line measurements of the size distribution and chemical composition. This involves the development of new real-time and in-situ characterization techniques. 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. Thus, experiments aiming to determine the elemental composition 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 (1064? nm) 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 and oxygen were recorded using an echelle spectrometer coupled to an intensified charge-coupled device detector. The recorded spectra were then compared to theoretical spectra calculated for a plasma in the Local Thermodynamic Equilibrium [1]. We found that the stoichiometric composition of the alumina particles can be deduced from the best agreement between measured and computed spectra if the experimental conditions are properly chosen. In particular, the background gas and the gate delay have to be optimized to perform precise measurements. These preliminary experiments show that the elemental composition of alumina particles can be measured via LIBS without any requirement of calibration with standards.