Laser-induced Breakdown Spectroscopy (libs) for Elemental Composition Analysis of Aerosols

Laser-induced Breakdown spectroscopy was applied for the elemental analysis of aerosols. Particular care has been taken in optimizing the time resolution of the LIBS spectra and the detection limit of the technique has been evaluated for Na, Ca and Mg atoms. The applicability of the technique has been extended to the case of fly ash generated from coal combustion. Introduction The analysis of aerosol particles may involve a wide range of applications such as monitoring of combustion processes and effluent waste streams, particulate air pollution control and atmospheric sciences. Several aerosol measurement techniques have been devised encompassing a broad range of operating principles. In particular laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy technique that utilizes a laser-induced microplasma which functions as both the sample volume and the excitation source [1]. A pulsed laser beam is tightly focused in a particle source flow. The resulting optical breakdown decomposes and excites all species within the plasma volume. The light emission is characterized by a continuum spectrum containing discrete atomic emission lines. These lines, both neutral (I) and ionic (II) and the continuum emission decay with time, but persist strongly on the order of tens of microseconds. In general the continuum spectrum decay faster than the atomic lines allowing the possibility of detecting atomic lines with a good signal-to-noise ratio by adjusting the delay and the integration time of the detector gate. However from an analytical point of view all the quantitative aspects of LIBS are still under study to better understand the complex nature of the laser-sample and plasma-particle interaction processes which depend on the laser pulse characteristics, sample properties, space and time [2]. Applications of the LIBS technique cover a wide range of species and compounds indicating that nearly all elements of interest in aerosol analysis are readily accessible with LIBS. Fly ashes are the mayor combustion residues produced during the combustion of pulverized coal in thermoelectric power plants. It is a fine-grained, powdery particulate material that is carried off in the flue gas and usually collected by means of electrostatic precipitators and baghouses. However fine particulate can escape XXXIV Meeting of the Italian Section of the Combustion Institute 2 the collecting devices. Due to the strong signal intensity and relative simplicity of the LIBS apparatus, some applications of the LIBS technique for the analysis of the emissions of burning coal have been recently presented [3-4]. In this work, Laser-induced Breakdown Spectroscopy has been implemented for the detection of low concentration of elements of aerosols in air. Aerosols of sodium, calcium and magnesium were used for the optimization of the LIBS experimental set-up. Fly ashes analysis was used as practical application of the technique. Experimental set-up A LIBS experimental apparatus typically consists of a pulsed laser, optics, and a detection unit, mainly composed by a spectrometer and a detector. In the present work, a pulsed Nd:YAG laser (Quanta system, λ=1064 nm, 7 ns FWHM, 6Hz) was used to create the plasma by means of a 165-mm focal length lens. The plasma spectral emission was collected perpendicularly to the incident laser beam onto a fiber optic coupled to the detection unit. Two different detection units with different spectral resolution were implemented. A low resolution spectrograph (JY UFS 200) coupled with a gateable, intensified 1024 elements diode array (Tracor Northern, TN-6130-1) with related control unit (Tracor Northern, TN-1710) was used to record spectra from 231 nm to 847 nm. This allows to get an overview of the atomic emission over a wide range of wavelengths, but the lack in resolution limits the fine analysis of the atomic lines of the element of interest. Therefore, a high resolution spectrometer (Spex 1681 C, 1200 groove/nm, 0.2-nm resolution) coupled with an intensified CCD camera (Princeton EEV, 1152 x 298) was also employed. The intensified CCD detector was synchronized to the laser Q-switch in order to test different detector gates (delay and integration width). For each experimental condition 250 signals, each one made by an accumulation of 10 laser shots, were averaged in order to improve the signal to noise ratio. Particle source stream were generated using a typical pneumatic-type medical nebulizer. Aqueous solutions of NaCl, CaCl2 and MgCl2 were nebulized and introduced into an air flow to facilitate droplets vaporization and to produce a fine dispersion of metallic salt. For each element a range of solution concentrations (0.001-0.1M) has been tested in order to evaluate the detection limits of the technique. The atomic emission lines at 588.9 nm and 589.6 nm (Na), 393.6 nm and 396.8 nm (Ca) and 279.5 nm and 280.2 nm (Mg) have been investigated. Cl lines are out of the spectral range of our spectrometer. Results A typical LIBS spectrum of sodium recorded on time with the plasma emission is shown in Fig.1. As it can be observed, an intense continuum background due to the major Bremsstrahlung emission mechanism predominates, especially in the ultraviolet region. Strongest elemental spectral lines can be observed over this background. In the figure sodium atomic line at 588 nm is shown, together with the