UC San Diego

Laser-Induced Breakdown Spectroscopy (LIBS) is a fast and simple analytic technique that is finding use in a range of applications. With LIBS, a pulsed laser of sufficient energy is focused to a spot, at which point the material is ionized and heated to temperatures in excess of 15,000 K. Light from this laser-induced plasma is then collected into a spectrometer and analyzed. LIBS is an extremely versatile technique that may be used to analyze virtually any material and deployed in even the harshest environments. Recently, interest in the technique has been rapidly expanding and many new applications have been developed. One area that has received relatively little attention is the measurement of aerosols. In this work, attempts at two applications of LIBS for aerosol measurements are presented, along with a fundamental investigation of sources of uncertainty unique to aerosol measurements. In the first application, LIBS is examined as a potential method for detecting airborne biological agents. Laboratory measurements of some common biological agent simulants are compared to those of common, naturally occurring biological aerosol components (pollen and fungal spores) to determine the potential of LIBS for discriminating biological agents from natural background aerosols. In the second application, metal species in ambient particulate matter were continuously monitored during a week-long field trial. Average and temporally resolved concentrations of seven metals were measured, and individual particles containing multiple species were detected. After attempting several new applications, it was discovered that previously unknown effects unique to aerosol measurements introduce significant uncertainty into the LIBS signal, which substantially limit the usefulness of the technique. A fundamental investigation is presented that identifies the location of individual particles within the LIBS plasma as a significant source of uncertainty. Several experiments and modeling efforts demonstrate that the optical systems typically used in LIBS devices do not collect light uniformly across the plasma volume, and that these optical effects dominate other phenomena contributing to the final signal. Finally, methods of improving the performance of LIBS aerosol measurements by changing the optical design or introducing a particle triggering system to reduce the variation in particle location are discussed