Laser-induced breakdown spectroscopy for toxic metal emission measurements: Experimental considerations and oxygen quenching

Toxic metals form one of the most important fractions of PM 2.5 emitted from thermal processing units. Current and upcoming regulations place strict limits on toxic metals from many process streams. Laser-Induced Breakdown Spectroscopy (LIBS) is an emerging technique capable of sensitive elemental analysis at relatively high data rates, up to 20 Hz with sensitive intensified charge-coupled device (ICCD) detectors. In LIBS, a laser-induced microplasma is formed, with peak temperatures on the order of 10,000-25,000degreesC. The plasma vaporizes solid material, including complete vaporization of aerosol particles up to roughly 2-10 mum in diameter, and atomizes molecular species. Atoms in the hot plasma may become electrically excited, and relax as the plasma cools, emitting characteristic atomic emission lines that may be used as a measure of elemental concentrations. This paper focuses on the development of the LIBS technique, including experimental configurations, calibration and quantification, potential interferences, and applications. Particular focus will be placed on the application of LIBS to measurements of toxic metals, with an emphasis on the effects of variable oxygen concentration, as may be found in the stack of typical thermal processing units.