Frank C. De Lucia

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.

Laser-induced breakdown spectroscopy (LIBS) – an emerging field-portable sensor technology for real-time, in-situ geochemical and environmental analysis

Laser-induced breakdown spectroscopy (LIBS) is a simple spark spectrochemical sensor technology in which a laser beam is directed at a sample to create a high-temperature microplasma. A spectrometer/array detector is used to disperse the light emission and detect its intensity at specific wavelengths. LIBS has many attributes that make it an attractive tool for chemical analysis. A recent breakthrough in component development, the commercial launching of a small, high-resolution spectrometer, has greatly expanded the utility of LIBS and resulted in a new potential for field-portable broadband LIBS because the technique is now sensitive simultaneously to all chemical elements due to detector response in the 200 to 980 nm range with 0.1 nm spectral resolution. Other attributes include: (a) small size and weight; (b) technologically mature, inherently rugged, and affordable components; (c) in-situ analysis with no sample preparation required; (d) inherent high sensitivity; (e) real-time response; and (f) point sensing or standoff detection. LIBS sensor systems can be used to detect and analyse target samples by identifying all constituent elements and by determining either their relative or absolute abundances.

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.

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.

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.

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.

Man-Portable LIBS for Landmine Detection

Laser Induced Breakdown Spectroscopy (LIBS) is an emerging, minimally-destructive sensor technology for in-situ, real-time chemical species identification and analysis. The Army Research Laboratory has been engaged in LIBS analysis for over a decade and recently has been investigating the potential to apply broadband LIBS analysis to specific military problems, one of which is as a handheld, confirmatory sensor for landmine detection. Laboratory tests with a prototype man-portable LIBS system demonstrate a high degree of success in identifying landmine casings.

LIBS: a new versatile field-deployable real-time detector system with potential for landmine detection

Laser Induced Breakdown Spectroscopy (LIBS) is an atomic emission spectroscopic technique that utilizes a pulsed laser to create a microplasma on the target together with an array spectrometer to capture the transient light for elemental identification and quantification. LIBS has certain important characteristics that make it a very attractive sensor technology for military uses. Such attributes include that facts that LIBS (1) is relatively simple and straightforward, (2) requires no sample preparation, (3) generates a real-time response, and (4) only engages a very small sample (pg-ng) of matter in each laser shot and microplasma event, (5) has inherent high sensitivity, and (6) responds to all forms of unknowns, and, therefore, is particularly suited for the sensing of dangerous materials. Additionally, a LIBS sensor system can be inexpensive, configured to be man-portable, and designed for both in-situ point sensing and remote stand-off detection with distances of up to 20-25 meters. Broadband LIBS results covering the spectral region from 200-970 nm acquired at the Army Research Laboratory (ARL) under laboratory conditions for a variety of landmine casings and explosive materials. This data will illustrate the potential that LIBS has to be developed into a hand-deployable device that could be utilized as a confirmatory sensor in landmine detection. The concept envisioned is a backpack-size system in which an eyesafe micro-laser is contained in the handle of a deminers probe and light is delivered and collected through an optical fiber in the tapered tip of the probe. In such a configuration, analyses can be made readily by touching the buried object that one is interested in identifying.