The application of laser-induced breakdown spectroscopy (LIBS) for the analysis of aerosol systems is a challenging problem that entails a wide range of physical phenomena that are coupled to the ultimate analyte response. While the analysis of aerosol particles dates back to some of the earliest LIBS studies, the evolution of understanding of the many processes involved in transforming a solid particulate into a collection of dissociated atoms and ions necessary for atomic emission spectroscopy has largely occurred over the last decade. During this time, a number of studies have attempted to elucidate the physics of particle vaporization, dissociation and ionization, diffusion of heat and mass, and ultimately, the resulting atomic emission. This review seeks to summarize the evolution of thought with regard to LIBS-based analysis of aerosol systems and provide insight into future research directions.

The technique of laser-induced breakdown spectroscopy (LIBS) makes use of a laser-induced plasma to vaporize and dissociate a targeted material. Subsequent atomic emission from the analyte species within the laser-induced plasma forms the basis of LIBS as an analytical technique. One might trace the beginning of quantitative LIBS-based aerosol analysis to the pioneering work of Radziemski, Cremers, and colleagues (1). In their study, beryllium-rich aerosols were used for generation of calibration curves and subsequent calculation of detection limits. They demonstrated the viability of LIBS for detection and quantitative analysis of aerosol samples. In a following study, in which particle sizes were extended to the submicrometer-sized range, Radziemski and colleagues (2) generated calibration curves for three analytes — namely cadmium, lead, and zinc — which were characterized by initial linearity followed by various degrees of saturation at higher concentrations. The saturation effects were attributed to incomplete vaporization of the analyte-containing particles. Over the following two decades, the LIBS community often cited an upper size limit for complete particle vaporization of about 10 μm, some directly referring to the extensive work of Radziemski and Cremers (3,4), while others referred to such a limit as a more general guideline. Radziemski and Cremers (2) did report that the LIBS technique is useful as long as the aerosol particles are below about 10 μm in diameter and somewhat monodisperse, although no systematic study was ever undertaken to specifically quantify this issue. Only in more contemporary LIBS studies was the issue of an upper size limit for quantitative analysis of aerosol particles addressed directly (5,6), as discussed in more detail later.

An issue closely related to complete particle vaporization is the issue of the independence of the analyte response on the analyte source. In the larger analytical community, such a topic would be considered to fall under the label of matrix effects, although such terminology was not largely used in the LIBS community with regard to aerosol analysis. Semantics notwithstanding, the issue was addressed from the beginning, including in the early work by Radziemski and colleagues (2). An important finding of their study was the general agreement (within 10%) of lead atomic emission signals of comparable atomic lead concentrations when nebulizing either lead acetate, lead chloride, or lead nitrate. In addition to analysis of particle-derived analyte signals, researchers also have looked at the gas phase of the aerosol system, where the relative independence of analyte signals on the molecular source was reported in several studies (7,8). Specifically, Dudragne and colleagues (7) demonstrated that analyte signals for fluorine, chlorine, sulfur, and carbon scaled with the number of respective analyte atoms in the constituent molecules for a wide range of compounds, concluding that the parent molecules were fully dissociated in the laser-induced plasma. Tran and colleagues (8) verified that SF₆ and HF yielded identical fluorine atomic emission signals when the gas composition was adjusted to contain the same atomic fluorine concentration.

This discussion is by no means a comprehensive review of the early evolution of LIBS for aerosol analysis, but rather, is intended to set the stage for a more in-depth discussion of the physics of LIBS-based analysis of aerosol particles. However, the general concepts outlined here, namely, complete particle vaporization and independence of the LIBS signal on analyte source (that is, lack of significant matrix effects) — do provide the basis and motivation for a wide range of LIBS-based aerosol studies. In a number of applied studies, LIBS-based sensing has been implemented successfully for continuous on-line monitoring of emissions and industrial processes (9–24), for analysis of ambient air particulate matter (25–28), and for general aerosol systems and nanoparticles (29–33). LIBS also was used in conjunction with single-shot analysis to effectively sample and analyze aerosol populations using discrete particle analysis (12,25,34,35). Other studies have addressed the feasibility of LIBS for analysis of biological materials and bioaerosols (36–44). In view of the wide array of potential applications, important research issues regarding LIBS-based analysis of aerosol systems can be identified for further analysis, as discussed in the remainder of this review.