North American Symposium on Laser Induced Breakdown Spectroscopy (NASLIBS): introduction to feature issue.

This feature issue highlights the topics of the 2011 North American Symposium on Laser Induced Breakdown Spectroscopy (NASLIBS). These include LIBS application to Security and Forensic, Biomedical and Environmental, Liquid Analysis and Fundamentals of LIBS, Instrumentation/Commercialization, Fusion with LIBS, and New Frontiers.

Laser-induced breakdown spectroscopy for detection of explosives residues: a review of recent advances, challenges, and future prospects.

In this review we discuss the application of laser-induced breakdown spectroscopy (LIBS) to the problem of detection of residues of explosives. Research in this area presented in open literature is reviewed. Both laboratory and field-tested standoff LIBS instruments have been used to detect explosive materials. Recent advances in instrumentation and data analysis techniques are discussed, including the use of double-pulse LIBS to reduce air entrainment in the analytical plasma and the application of advanced chemometric techniques such as partial least-squares discriminant analysis to discriminate between residues of explosives and non-explosives on various surfaces. A number of challenges associated with detection of explosives residues using LIBS have been identified, along with their possible solutions. Several groups have investigated methods for improving the sensitivity and selectivity of LIBS for detection of explosives, including the use of femtosecond-pulse lasers, supplemental enhancement of the laser-induced plasma emission, and complementary orthogonal techniques. Despite the associated challenges, researchers have demonstrated the tremendous potential of LIBS for real-time detection of explosives residues at standoff distances.

Evaluation of femtosecond laser-induced breakdown spectroscopy for explosive residue detection.

Recently laser-induced breakdown spectroscopy (LIBS) has been investigated as a potential technique for trace explosive detection. Typically LIBS is performed using nanosecond laser pulses. For this work, we have investigated the use of femtosecond laser pulses for explosive residue detection at two different fluences. Femtosecond laser pulses have previously been shown to provide several advantages for laser ablation and other LIBS applications. We have collected LIBS spectra of several bulk explosives and explosive residues at different pulse durations and energies. In contrast to previous femtosecond LIBS spectra of explosives, we have observed atomic emission peaks for the constituent elements of explosives - carbon, hydrogen, nitrogen, and oxygen. Preliminary results indicate that several advantages attributed to femtosecond pulses are not realized at higher laser fluences.

Standoff detection of chemical and biological threats using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) is a promising technique for real-time chemical and biological warfare agent detection in the field. We have demonstrated the detection and discrimination of the biological warfare agent surrogates Bacillus subtilis (BG) (2% false negatives, 0% false positives) and ovalbumin (0% false negatives, 1% false positives) at 20 meters using standoff laser-induced breakdown spectroscopy (ST-LIBS) and linear correlation. Unknown interferent samples (not included in the model), samples on different substrates, and mixtures of BG and Arizona road dust have been classified with reasonable success using partial least squares discriminant analysis (PLS-DA). A few of the samples tested such as the soot (not included in the model) and the 25% BG:75% dust mixture resulted in a significant number of false positives or false negatives, respectively. Our preliminary results indicate that while LIBS is able to discriminate biomaterials with similar elemental compositions at standoff distances based on differences in key intensity ratios, further work is needed to reduce the number of false positives/negatives by refining the PLS-DA model to include a sufficient range of material classes and carefully selecting a detection threshold. In addition, we have demonstrated that LIBS can distinguish five different organophosphate nerve agent simulants at 20 meters, despite their similar stoichiometric formulas. Finally, a combined PLS-DA model for chemical, biological, and explosives detection using a single ST-LIBS sensor has been developed in order to demonstrate the potential of standoff LIBS for universal hazardous materials detection.

Multivariate analysis of standoff laser-induced breakdown spectroscopy spectra for classification of explosive-containing residues.

A technique being evaluated for standoff explosives detection is laser-induced breakdown spectroscopy (LIBS). LIBS is a real-time sensor technology that uses components that can be configured into a ruggedized standoff instrument. The U.S. Army Research Laboratory has been coupling standoff LIBS spectra with chemometrics for several years now in order to discriminate between explosives and nonexplosives. We have investigated the use of partial least squares discriminant analysis (PLS-DA) for explosives detection. We have extended our study of PLS-DA to more complex sample types, including binary mixtures, different types of explosives, and samples not included in the model. We demonstrate the importance of building the PLS-DA model by iteratively testing it against sample test sets. Independent test sets are used to test the robustness of the final model.

Detection of indoor biological hazards using the man-portable laser induced breakdown spectrometer.

The performance of a man-portable laser induced breakdown spectrometer was evaluated for the detection of biological powders on indoor office surfaces and wipe materials. Identification of pure unknown powders was performed by comparing against a library of spectra containing biological agent surrogates and confusant materials, such as dusts, diesel soot, natural and artificial sweeteners, and drink powders, using linear correlation analysis. Simple models constructed using a second technique, partial least squares discriminant analysis, successfully identified Bacillus subtilis (BG) spores on wipe materials and office surfaces. Furthermore, these models were able to identify BG on materials not used in the training of the model.

Use of laser induced breakdown spectroscopy in the determination of gem provenance: beryls.

The provenance of gem stones has been of interest to geologists, gemologists, archeologists, and historians for centuries. Laser induced breakdown spectroscopy (LIBS) provides a minimally destructive tool for recording the rich chemical signatures of gem beryls (aquamarine, goshenite, heliodor, and morganite). Broadband LIBS spectra of 39 beryl (Be(3)Al(2)Si(6)O(18)) specimens from 11 pegmatite mines in New Hampshire, Connecticut, and Maine (USA) are used to assess the potential of using principal component analysis of LIBS spectra to determine specimen provenance. Using this technique, beryls from the three beryl-bearing zones in the Palermo #1 pegmatite (New Hampshire) can be recognized. However, the compositional variation within this single mine is comparable to that in beryls from all three states. Thus, a very large database with detailed location metadata will be required to routinely determine gem beryl provenance.

LIBS for landmine detection and discrimination.

The concept of utilizing laser-induced breakdown spectroscopy (LIBS) technology for landmine detection and discrimination has been evaluated using both laboratory LIBS and a prototype man-portable LIBS systems. LIBS spectra were collected for a suite of landmine casings, non-mine plastic materials, and "clutter-type" objects likely to be present in the soil of a conflict area or a former conflict area. Landmine casings examined included a broad selection of anti-personnel and anti-tank mines from different countries of manufacture. Other materials analyzed included rocks and soil, metal objects, cellulose materials, and different types of plastics. Two "blind" laboratory tests were conducted in which 100 broadband LIBS spectra were obtained for a mixed suite of landmine casings and clutter objects and compared with a previously-assembled spectral reference library. Using a linear correlation approach, "mine/no mine" determinations were correctly made for more than 90% of the samples in both tests. A similar test using a prototype man-portable LIBS system yielded an analogous result, validating the concept of using LIBS for landmine detection and discrimination.

Laser-induced breakdown spectroscopy analysis of complex silicate minerals--beryl.

Beryl (Be3Al2Si6O18) is a chemically complex and highly compositionally variable gem-forming mineral found in a variety of geologic settings worldwide. A methodology and analytical protocol were developed for the analysis of beryl by laser-induced breakdown spectroscopy (LIBS) that minimizes the coefficient of variance for multiple analyses of the same specimen. The parameters considered were laser energy/pulse, time delay and crystallographic orientation. Optimal analytical conditions are a laser energy/pulse of 102 mJ and a time delay of 2 micros. Beryl compositions measured parallel and perpendicular to the c axis were identical within analytical error. LIBS analysis of 96 beryls from 16 countries (Afghanistan, Brazil, Canada, China, Colombia, India, Ireland, Italy, Madagascar, Mexico, Mozambique, Namibia, Norway, Russia, Tanzania and United States), Antarctica, and ten US states (AZ, CA, CO, CT, ID, ME, NC, NH, NM and UT) were undertaken to determine whether or not LIBS analysis can be used to determine the provenance of gem beryl.

Temporal evolution of the laser-induced breakdown spectroscopy spectrum of aluminum metal in different bath gases.

The spectral emission of gas-phase aluminum and aluminum oxide was measured during and immediately after exposure of a bulk-aluminum sample to a laser-induced spark produced by a focused, pulsed laser beam (Nd:YAG, 10-ns pulse duration, 35 mJ/pulse, lambda = 1064 nm). The spectral emission was measured as a function of time after the onset of the laser pulse, and it was also measured in different bath gases (air, nitrogen, oxygen, and helium).

Kinetic modeling of the laser-induced breakdown spectroscopy plume from metallic lead.

We report initial results of a study aimed toward developing a computational fluid dynamics (CFD) model to simulate the laser-induced breakdown spectroscopy (LIBS) plume for the purpose of understanding the physical and chemical factors that control the LIBS signature. The kinetic model developed for modeling studies of the LIBS plume from metallic lead includes a set of air reactions and ion chemistry as well as the oxidization, excitation, and ionization of lead atoms. At total of 38 chemical species and 220 reactions are included in the model. Experimental measurements of the spatial and temporal dependence of a number of lead emission lines have been made of the LIBS plume from metallic lead. The mechanism of generation of excited Pb states in the LIBS plume is analyzed through kinetic modeling and sensitivity analysis. Initial CFD model results for the LIBS plume are presented.

Laser-induced breakdown spectroscopy analysis of energetic materials.

A number of energetic materials and explosives have been studied by laser-induced breakdown spectroscopy (LIBS). They include black powder, neat explosives such as TNT, PETN, HMX, and RDX (in various forms), propellants such as M43 and JA2, and military explosives such as C4 and LX-14. Each of these materials gives a unique spectrum, and generally the spectra are reproducible shot to shot. We observed that the laser-produced microplasma did not initiate any of the energetic materials studied. Extensive studies of black powder and its ingredients by use of a reference spectral library have demonstrated excellent accuracy for unknown identification. Finally, we observed that these nitrogen- and oxygen-rich materials yield LIBS spectra in air that have correspondingly different O:N peak ratios compared with air. This difference can help in the detection and identification of such energetic materials.

Laser-induced breakdown spectroscopy of bacterial spores, molds, pollens, and protein: initial studies of discrimination potential.

Laser-induced breakdown spectroscopy (LIBS) has been used to study bacterial spores, molds, pollens, and proteins. Biosamples were prepared and deposited onto porous silver substrates. LIBS data from the individual laser shots were analyzed by principal-components analysis and were found to contain adequate information to afford discrimination among the different biomaterials. Additional discrimination within the three bacilli studied appears feasible.

Fourier-transform laser spectroscopy.

The development of microphotonic sensors based on Fourier-transform laser spectroscopy (FT-LS) is discussed. The application demonstrated is for measurement of vapors from the hydrocarbon fuels JP-8, diesel fuel, and gasoline. The two-laser prototype FT-LS sensor used for our research employs distributed-feedback lasers in the near-infrared spectral region (1.3- and 1.7-microm wavelength). An extension of this research to multilaser arrays is discussed. We believe that this is the first measurement of middle-distillate fuel-vapor concentrations using this optical mixing technique.