Analysis of Aerosols in a flow cell and in a low-pressure Radio-Frequency Plasma cell using LIBS for process control and workplace surveillance
Abstract
New issues related to process control and workplace surveillance accompany the emergence of nanotechnology industry. The quality control of nanoparticles in industrial production and their monitoring for workplace surveillance purposes require on-line determination of concentration and chemical composition. This involves the development of new real-time and in-situ characterization techniques. In this context, laser-induced breakdown spectroscopy (LIBS) is a promising technique in both process control and workplace surveillance. Thus, experiments with two different approaches of the LIBS technique have been carried out. The first approach used a flow cell to determine the elemental composition of an aerosol with a calibration-free (CF) procedure. The second one used a low-pressure radio-frequency (RF) cell as a particle trap to analyze aerosols containing nanoparticles. The experimental setup consists of the aforementioned analysis cells coupled to a LIBS optical setup. In both cases, studied particles were injected using an inert gas (argon/helium). The flow cell setup used an aerosol flow of alumina (Al2O3) micrometric particles for the analysis. The LIBS-RF setup used RF plasma generated with the inert gas (argon) at a low- pressure (2 mbar) to concentrate and trap particles in levitation inside the RF plasma. In both cases, the optical setup consists of focusing laser pulses of 5 ns duration originating from a Q- switched Nd:YAG laser (1064 nm) on the aerosol inside each analysis cell where the plasma was generated. When resorting to CF-LIBS, the recorded spectra were compared to theoretical spectra calculated for a plasma in the Local Thermodynamic Equilibrium1. 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 without any requirement of calibration with standards. The use of the low-pressure RF plasma cell as a particle trap improved the sampling by concentrating particles. The LIBS-RF signal was enhanced by eliminating the plasma continuum thanks to the low-pressure. Elements difficult to analyze in conventional conditions such as C,H,O,N were detected with a very good signal-to-noise ratio. New studies aiming to validate the CF-LIBS with other powders (CaTiO3 and MnTiO3) are currently underway and new experiments with the LIBS-RF aiming to determine the particle density and the elemental concentration in an aerosol are scheduled.