Applications of Laser-Induced Breakdown Spectrometry

Abstract

INTRODUCTION When a high-powered laser beam is focused onto a small area or spot of a solid surface, the temperature of the locally heated region rises rapidly to the vaporization temperature of the solid material and an optically induced plasma, frequently called a laser-induced plasma (LIP) or laser-ablated plasma (LAP) or laser spark is formed at the surface. The plasma will be formed when the laser power density exceeds the breakdown threshold value of the solid surface. Although different materials have different breakdown thresholds, an optical plasma is produced when the laser power density exceeds several megawatts per centimeter squared (106 - 109 W/cm2). This plasma has been used for sampling, atomization, excitation, and ionization in analyhcal atomic spectroscopy. It has also been frequently used and proposed as a source for atomic emission spectrometry (AES). In this case the technique is most ofien referred to as laser microprobe optical emission spectrometry (LM-OES) developed by Brech and Cross [1] in early nineteen-sixties or more recently called laser-induced breakdown spectrometry (LIBS) [2,3]. Generally, this analyhcal technique involves two steps; the pulsed focused laser beam directed into a gaseous sample or the surface of a solid or liquid, to produce a transient LIP, followed by the measurement of a characteristic atomic emission signal related to some species present in the plasma. The LIP formed is tightly focused and consists of vaporized atoms, ions, electrons, and molecular fragments. The application of LIBS for direct spectrochemical analysis is a rapidly growing field ranging from the detection of atmospheric pollutants to monitoring of material production processes, and even to “clean-room” technology. Laser ablation techniques have also been applied for solid sample introduction into other plasma sources [4–9]. In recent years, the powerful technique of LIBS as an analytical tool has been recognized by a number of research groups, and has led to an increasing number of publications on the applications of LIBS both in the laboratory and in industry. This growing success of LIBS is a result of thorough research carried out to understand the related plasma physical processes, aided by marked improvements in laser systems and photodetector technology.