Test of a stand-off laser-induced breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces Cristina Lo´pez-Moreno, a Santiago Palanco, a J. Javier Laserna,* a Frank DeLucia Jr, b Andrzej W. Miziolek, b Jeremy Rose, c Roy A. Walters c and Andrew I. Whitehouse d Received 8th June 2005, Accepted 19th October 2005 First published as an Advance Article on the web 10th November 2005 DOI: 10.1039/b508055j The detection and characterization of energetic materials at distances up to 45 m using stand-off laser induced breakdown spectroscopy (LIBS) has been demonstrated. A field-portable open-path LIB spectrometer working under a coaxial configuration was used. A preliminary study allowed choosing a single-pulse laser source over a double-pulse system as the most suitable source for the stand-off analysis of organic samples. The C 2 Swan system, as well as the hydrogen, oxygen and nitrogen emission intensity ratios were the necessary parameters to identify the analyte as an organic explosive, organic non-explosive and non-organic samples. O/N intensity ratios of 2.9 and 2.16 with relative standard deviations of 4.03% and 8.36% were obtained for 2,4-dinitrotoluene and aluminium samples, respectively. A field test with known samples and a blind test were carried out at a distance of 30 m from the sample. Identification of energetic compounds in such conditions resulted in 19 correct results out of 21 samples. Introduction Detection of energetic materials requires the development of new analytical techniques. 1,2 A suitable analytical tool should be able to differentiate these compounds from innocuous substances with a reasonable level of confidence. Although several systems are commercially available, until now, no instrument can afford detectability for the wide range of energetic materials in any quantity and in any form without false alarms being detected. Radiation-based and vapor-based are the main techniques currently used in the detection of explosives. In the first group, neutron activation analysis can be highlighted. This technique is based on the measurement of the g emission generated in a material in response to an activation by thermal neutrons. The intensity and the spatial distribution of the g-photons are indicative of the presence of N in the material. Several of the most usual explosives have a high nitro group content. However, there are highly nitrogen- containing polymers such as melamine or the polyamides which often induce false positives. Among vapor-based tech- niques, it is worth mentioning ion mobility spectrometry owing to its high sensitivity to volatile compounds evaporating from most explosives. However, the biggest disadvantage of these vapor-based techniques is their limitation to organic and easily vaporizable compounds. 1 The techniques used up to now still require approaching the sample in order to perform the analysis which entails a risk for the operator. The fact that dogs remain the most effective explosive detectors nowadays is highly indicative that explora- tion of new analytical techniques is critical. In this sense, remote analytical techniques are the only ones that offer real-time results maintaining a security distance from the sample and avoiding risks for the operator. Laser-induced breakdown spectroscopy (LIBS) has been successfully used for the laboratory detection and identification of chemical and biological warfare agents, explosives and other hazards. 3–5 LIBS possesses many desirable attributes for a fast field- portable sensor system. The portability 6,7 and the remote capabilities 8,9 of LIBS make this technique the most suitable for hazardous materials in the field. However, energetic ma- terials are commonly organic compounds and LIBS is mainly an elemental technique owing to the high-energies related to the focused short laser pulses. Identification of organic compounds with LIBS has been reported in several papers. 10–20 Anzano et al. 10 proved the feasibility of a data correlation method 11 for the identification of organic polymers. More often, oxygen, nitrogen and hydro- gen emission lines and a number of molecular bands are used for this task. The analysis of molecular bands 12,13 is focused on the detection of CN molecular violet bands at 386.17 nm, 387.14 nm, and 388.34 nm and C 2 carbon Swan bands at 516.52 nm. It is well known that the intensity of the Swan system is proportional to the concentration of the carbon dimmer in the excited state while the CN bands emission could be also due to the CN generation in the ambient air. Thus, only the measurement of the C 2 bands is reliable for the analysis in the open atmosphere. Although the raw atomic emission has been used for the analysis of organics, 12,14–16 the a Department of Analytical Chemistry, Faculty of Sciences, University of Ma ´laga, E-29071, Spainlaserna@uma.es b US Army Research Laboratory, AMSRD-ARL-WM-BD, Aberdeen Proving Ground, MD 21005-5069, USA. E-mail: miziolek@arl.army.mil c Ocean Optics, Inc., Winter Park, FL 32792-6819, USA. E-mail: royW@oceanoptics.com d Applied Photonics Ltd, Unit 8, Carleton Business Park, Skipton, North Yorkshire, UK BD23 2DE. E-mail: andy.whitehouse@appliedphotonics.co.uk This journal is  c The Royal Society of Chemistry 2006 J. Anal. At. Spectrom., 2006, 21, 55–60 | 55 PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry