In laser-induced breakdown spectroscopy (LIBS), a focused laser pulse is used to ablate material from a surface and form a laser plasma that excites the vaporized material. Geometric factors, such as the distance between the sample and the focusing lens and the method of collecting the plasma light, can greatly influence the analytical results. To obtain the best quantitative results, one must consider this geometry. Here we report the results of an investigation of the effect of sampling geometry on LIBS measurements. Diagnostics include time-resolved spectroscopy and temporally and spectrally resolved imaging using an acousto-optic tunable filter (AOTF). Parameters investigated include the type of lens (cylindrical or spherical) used to focus the laser pulse onto the sample, the focal length of the lens (75 or 150 mm), the lens-to-sample distance (LTSD), the angle-of-incidence of the laser pulse onto the sample, and the method used to collect the plasma light (lens or fiber-optic bundle). From these studies, it was found that atomic emission intensities, plasma temperature, and mass of ablated material depend strongly on the LTSD for both types of lenses. For laser pulse energies above the breakdown threshold for air, these quantities exhibit symmetric behavior about an LTSD approximately equal to the back focal length for cylindrical lenses and asymmetric behavior for spherical lenses. For pulse energies below the air breakdown threshold, results obtained for both lenses display symmetric behavior. Detection limits and measurement precision for the elements Be, Cr, Cu, Mn, Pb, and Sr, determined with the use of 14 certified reference soils and stream sediments, were found to be independent of the lens used. Time-resolved images of the laser plasma show that at times >5 μs after plasma formation a cloud of emitting atoms extends significantly beyond the centrally located, visibly white, intense plasma core present at early times (<0.3 μs). It was determined that, by collecting light from the edges of the emitting cloud, one can record spectra using an ungated detector (no time resolution) that resemble closely the spectra obtained from a gated detector providing time-resolved detection. This result has implications in the development of less expensive LIBS detection systems.

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Laser-induced breakdown spectroscopy (LIBS) was used in a blind study to successfully differentiate bacterial pathogens, both species and strain. The pathogens used for the study were chosen and prepared by one set of researchers. The LIBS data were collected and analyzed by another set of researchers. The latter researchers had no knowledge of the sample identities other than that (1) the first five of fifteen samples were unique (not replicates) and (2) the remaining ten samples consisted of two replicates of each of the first five samples. Using only chemometric analysis of the LIBS data, the ten replicate bacterial samples were successfully matched to each of the first five samples. The results of this blind study show it is possible to differentiate the bacterial pathogens Escherichia coli, three clonal methicillin-resistant Staphylococcus aureus (MRSA) strains, and one unrelated MRSA strain using LIBS. This is an important finding because it demonstrates that LIBS can be used to determine bacterial pathogen species within a defined sample set and can be used to differentiate between clonal relationships among strains of a single multiple-antibiotic-resistant bacterial species. Such a capability is important for the development of LIBS instruments for use in medical, water, and food safety applications.

The rapid detection of biological contaminants, such as Escherichia coli O157:H7 and Salmonella enterica , on foods and food-processing surfaces is important to ensure food safety and streamline the food-monitoring process. Laser-induced breakdown spectroscopy (LIBS) is an ideal candidate technology for this application because sample preparation is minimal and results are available rapidly (seconds to minutes). Here, multivariate regression analysis of LIBS data is used to differentiate the live bacterial pathogens E. coli O157:H7 and S. enterica on various foods (eggshell, milk, bologna, ground beef, chicken, and lettuce) and surfaces (metal drain strainer and cutting board). The type (E. coli or S. enterica) of bacteria could be differentiated in all cases studied along with the metabolic state (viable or heat killed). This study provides data showing the potential of LIBS for the rapid identification of biological contaminants using spectra collected directly from foods and surfaces.