Identifying Explosive Materials and Analyzing Post-Explosion Residues

The prevention of accidental and intentional explosions is an increasingly prominent global issue that depends on the detection of explosive materials and analysis of post-explosion residues. However, detection of explosive materials can be dangerous due to the imminent threat of explosion, and the potential presence of toxic materials. Therefore, techniques for detecting explosive materials must be fast, efficient and able to operate from a safe distance.

Recently, advanced spectroscopic techniques have begun to allow rapid detection of explosive materials in a range of situations. This article briefly outlines some of the challenges and recent developments in identifying explosive materials. The 2017 Pittcon Conference & Expo (March 5–9, Chicago, IL, USA) is the ideal place for researchers to learn more about the techniques discussed in this article and many more.

Challenges Associated with the Identification of Explosive Materials

Explosive materials usually combine a hydrocarbon-based fuel component, and a nitrogen or oxygen based trigger component.1 There are over 150 known explosive materials in use today, including many common materials with multiple uses.2 Therefore, a major challenge associated with identifying explosive materials is avoiding false positives from common materials that are not being employed in an explosive material. In general, this can be achieved by determining the relative amounts of oxygen, nitrogen and other elements present in a material.

The detection of improvised explosive devices (IEDs), which are currently of major concern globally, presents further challenges. IEDs by definition contain non-traditional components that may be difficult to detect. In response to concern surrounding the use of IEDs,

NATO has developed a counter-IED action plan,3 and a recent report by the Office of Naval Research highlighted the need for improved rapid detection of explosive materials in the field.4

The Rise in Handheld Devices for Identifying Explosive Materials

Traditional spectroscopic techniques often require a trained chemist and can be time-consuming. Therefore, the development of spectroscopic methods that can identify explosive materials quickly and can be used by a non-expert outside a laboratory is essential. Recently, progress towards handheld devices for the detection of explosives has been accelerating.

Handheld devices using optical spectroscopy are seen as the most promising techniques for the detection of explosive materials. Many optical techniques have been adapted into portable forms suitable for explosive material detection from a safe distance including Raman spectroscopy,8,9 laser-induced breakdown spectroscopy (LIBS),10 laser-induced fluorescence (LIF),11 and cavity ringdown spectroscopy (CRDS).12 Each optical spectroscopy technique has its own specific advantages and disadvantages.

LIBS for Identifying Explosive Materials

LIBS is a spectroscopic technique that excites a sample to the point of breakdown into its constituent parts, forming a microplasma. The light emitted from the microplasma is then used to determine the molecular components of the sample. LIBS is a surface technique that requires no sample preparation and provides analysis in a few seconds.13 To overcome background signals resulting from nitrogen and oxygen in the air, researchers at the Army Research Laboratory have developed a modified version of LIBS known as double pulse LIBS.14 Double pulse LIBS employs laser pulses to displace surrounding gases and create an area of reduced pressure, and then excite the sample material with minimal interference from atmospheric gases. Double pulse LIBS has been shown to identify explosive materials effectively. The Army Research Laboratory have been working on ‘suitcase LIBS,' with LIBS technology contained in a 9 kg suitcase for field applications.

Raman Spectroscopy for Identifying Explosive Materials

Raman spectroscopy works on the principle that incident light is inelastically scattered and shifted in frequency due to molecular vibrations. Raman spectroscopy provides noninvasive analysis at safe distances and the ability to detect small concentrations of materials. Challenges for explosive material identification using Raman spectroscopy include weak signals, contamination and changing environments.15

Researchers at Caltech have been investigating the detection of explosive materials using ‘2D correlation spectroscopy’ where Raman spectroscopy is combined with changes in temperature.16,17 As the Raman peaks for explosive materials decrease with increasing temperature, 2D correlation spectroscopy can successfully distinguish the Raman spectra of explosive materials from static background contaminants, therefore reducing the chance of false positives. Ongoing work at Caltech is focused on optimizing optics and reducing integration times to make this technique portable and allow rapid analysis.

LIF for Identifying Explosive Materials

LIF is a spectroscopic technique whereby a molecule is excited to a higher energy state, then relaxes back to its ground state by spontaneous light emission.18 Researchers at MIT have been developing a technique involving photodissociation followed by LIF (PD-LIF) for detecting explosive residues at distances of 10 m.11 Nitrogen-rich molecules, such as those used in explosives, dissociate in UV light producing nitric oxide with excess vibrational energy. The excess vibrational energy of the NO originating from dissociated explosive materials allows it to be distinguished from background NO using LIF. LIF-based techniques offer several advantages including the use of low power eye-safe lasers, strong returning signal, and strong specificity. Ongoing research at MIT is centered on scaling up LIF lasers and optics for large area observation.

Conclusion

The identification of explosive materials and analysis of post-explosion residues is a complex and challenging task that relies upon the development of new spectroscopic techniques. Researchers around the world are working on adapting spectroscopic techniques for the efficient detection of explosive materials in the field, and ongoing research is centered on miniaturizing devices and increasing standoff distances, with the aim of producing handheld devices for field applications. Pittcon 2017 will feature many experts in the field of explosives detection, and dozens of exhibiting companies including Thermo Scientific, Rigaku, and Metrohm, making it a must-attend event for researchers interested in explosive material analysis.

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