Chase A. Munson

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

AbstractIn 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. FigureThis review discusses the application of laser-induced breakdown spectroscopy (LIBS) to the problem of explosive residue detection. LIBS offers the capability for real-time, standoff detection of trace amounts of residue explosives on various surfaces

Strategies for residue explosives detection using laser-induced breakdown spectroscopy

The ability to detect trace amounts of explosives and their residues in real time is of vital interest to Homeland Security and the military. Previous work at the US Army Research Laboratory (ARL) demonstrated the potential of laser-induced breakdown spectroscopy (LIBS) for the detection of energetic materials. Our recent efforts have been focused on improving the sensitivity and selectivity of LIBS for residue explosives detection and on extending this work to the standoff detection of explosive residues. One difficulty with detecting energetic materials is that the contribution to the oxygen and nitrogen signals from air can impede the identification of the explosive material. Techniques for reducing the air entrainment into the plasma—such as using an argon buffer gas or a collinear double-pulse configuration—have been investigated for this application. In addition to the laboratory studies, ARL’s new double-pulse standoff system (ST-LIBS) has recently been used to detect explosive residues at 20 m. The efficacy of chemometric techniques such as linear correlation, principal components analysis (PCA), and partial least squares discriminant analysis (PLS-DA) for the identification of explosive residues is also discussed. We have shown that despite the typical characterization of LIBS as an elemental technique, the relative elemental intensities in the LIBS spectra are representative of the stoichiometry of the parent molecules and can be used to discriminate materials containing the same elements. Simultaneous biohazard and explosive residue discrimination at standoff distances has also been demonstrated.

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.

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.

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).

Laser-Based Detection Methods for Explosives

Publisher Summary Lasers offer multiple approaches for explosive detection that are not possible with other techniques. In general, these can be separated into two types: those based on the unique properties of lasers for long-distance propagation of intense energy and those that are based on the actual molecular and atomic spectroscopy, and as such utilize the high wavelength specificity that most lasers offer. Of course, the field of laser explosive detection is somewhat new, given the fact that lasers were invented fairly recently in 1958. As such, it is fair to say that laser explosive detection is still a work in progress, with much having been discovered in recent years and still more to be discovered in the near future, particularly as more exotic laser sources, for example, fem to second lasers, become more common, less expensive, more rugged, and generally more readily available. The expected dramatic improvements in probability of detection and reduction of false alarm rates suggest that laser-based explosive detection methods may evolve into a major new technology area in the next 1–3 years.

Specific biological agent taggants

The preliminary data presented here suggests that direct coating of biological agent with DNA capture elements and organic semiconductor (DALM) with chelated rare earths such as scandium, europium or neodymium can be used to track the agent, even when the biological components have been subsequently destroyed. The use of these three taggant components in conjunction with each other affords the opportunity to determine the presence of the biological agent by several methods---laser induced plasma spectroscopy, thermochemiluminescence, mass spectroscopy, polymerase chain reaction (PCR; if the primers are left on the DCEs or the agents own DNA is used as the source of the amplicon). The specific DCE-labeling or PCR allows for confirmation of physical measurement results as specific to the agent.

Laser-induced breakdown spectroscopy (LIBS): an emerging field-portable sensor technology for real-time chemical analysis for military, security and environmental applications

Laser-induced breakdown spectroscopy (LIBS) is an emerging atomic emission spectroscopic technique that offers the prospect highly- selective, sensitive, and of real-time detection and analysis of both natural and man-made materials. Because LIBS is simultaneously sensitive to all chemical elements due to detector response in the 200-980nm range with 0.1 nm spectral resolution, the technique has many attributes that make it an attractive tool for a variety of military, security, and environmental applications.

Filament ablation of opaque solid material

Filamentation of femtosecond laser pulses defy diffraction and exist for many meters. The filament contains the concentrated laser pulse and a plasma column in its wake. We study filament ablation of metal and polymer surfaces.

Progress in Standoff LIBS Detection and Identification of Residue Materials

The LIBS group at ARL/APG continues to advance the use of Laser Induced Breakdown Spectroscopy (LIBS) for standoff detection and identification of various residue materials, both hazardous and benign. Progress will be presented.