Recent Advances in Trace Explosives Detection

Sens Imaging (2007) 8:9–38 DOI 10.1007/s11220-007-0029-8 ORIGINAL PAPER

Recent Advances in Trace Explosives Detection Instrumentation
David S. Moore

Received: 3 January 2007 / Accepted: 6 March 2007 / Published online: 26 May 2007 © Springer Science+Business Media, LLC 2007

Abstract There has been a huge increase in instrument development for trace detection of explosives in the past 3 years. This is especially true for methods that can be used at a stand off distance, driven by the frightening increase in the use of improvised explosive devices in both suicide and road side bombings. This review attempts to outline and enumerate these recent developments, with details about the improvements made as well as where further improvements might come. Keywords Reviews Trace detection · Trace analysis · Explosives · Trace analytical instrumentation ·

1 Introduction The inventiveness and creativity of those that would do the civilized world harm are seemingly limitless. This fact has been true throughout history; today is no exception. While civilized people might have difficulty understanding their enemies’ motivation, they can and must use their own creativity to proactively conceive adequate defenses. The most recent alarming increase in number and violence of terrorist bombings has made the task of standoff detection of improvised explosive devices extremely urgent. Yet, because of the variety of explosive materials available, cleverness of packaging, variability of venue, and the (mostly) low vapor pressures of explosives, the task of detection is extremely difficult. This review is intended to highlight recent advances in analytical instrumentation and methodology applicable to trace, vapor, and stand-off explosives detection. It is also intended to compare current capabilities to what is necessary for field use. As was the case with my earlier review [1], the focus here will be on results published in the archival scientific literature, via both peer reviewed journals and proceedings volumes (and also some National Laboratory reports), rather than vendor information.

D. S. Moore (B ) Shock and Detonation Physics Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA e-mail: [email protected]

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2 Properties of Explosives The attributes of an explosion are: a chemically or structurally unstable molecule or mixture, a rapid rate of reaction, a large amount of heat generated, and a large fraction of gaseous products so that the reaction produces large changes in pressure. These attributes are also characteristic of fuels and oxidizers; what differentiates explosives is their ability to not react until initiated by shock or heat applied to a small volume, which expands rapidly through the material. A rapid decomposition (deflagration) without formation of a shock wave is characteristic of a low explosive. Lack of shock formation is ideal (even critical) for use of energetic materials as propellants. High explosives support a leading shock (the detonation front) at velocities from 1 to 9 km/s. In certain circumstances, a high explosive only reacts at low order. Such circumstances might include small diameter charges (below the failure diameter) or low density. The sensitivity to impact, friction, and heat differentiates primary from secondary explosives. While a typical high explosive has fuel and oxidizer in the same molecule, other materials also yield large amounts of energy and gas in a very short time and have therefore been exploited by terrorists. Even primary explosives (those whose initiation characteristics make them difficult to handle safely) have been used (e.g., TATP, HMTD, NG). Intimate mixtures of fuels and oxidizers are common (e.g., ANFO, slurries, black powder). Oxley and Smith have recently provided a relevant overview of the properties of peroxide explosives [2]. The number and kinds of explosive materials utilized recently has necessitated more basic studies on their properties, including those properties relevant to their detection. In particular, a number of new studies of vapor pressure have been published [2–6]. These have been incorporated into Fig. 1, revised and updated from Figure 1 of Reference 1. Some common explosives missing from the earlier figure have been added (NM, H2 O2 , TATB, DATB, NG, TNM) [3–6]. Fortunately, most of the newly utilized materials have significant vapor pressures, reducing the difficulty of vapor phase detection. On the other hand, traces of explosive materials on the exterior of packaging disappear more quickly for higher vapor pressure materials [7,8], so that methods relying on swipes or surface treatments may have a time dependence not seen for lower vapor pressure materials. Studies of other properties specific to detection methods are discussed in each section below. There have been a number of studies of source term dynamics and vapor transport. The transport of explosive vapors from buried landmines (which could be equated to buried IEDs) has been modeled as a diffusion process, with detection scenarios based on statistical models [9]. The diffusion process can be strongly perturbed by turbulent atmospheric flows, causing large fluctuations in concentration with space and time. In addition, packaging plays an extremely important role in concealing explosives [1]. Information on common methods used to mask the presence of IEDs has been presented in a concise paper by Turecek [10].

3 Recent Reviews A large number of workshops and conferences have been held in the past 3 years that are relevant to trace detection of explosives [see Conference References]. New terahertz technology and methods were presented at SPIE conferences on terahertz for military and security applications. The coupling of sensors with command, control, communications, and intelligence technologies was the subject of SPIE meetings in 2004, 2005, and 2006. Many overlapping technologies were discussed in the SPIE meetings on detection and remediation technologies for mine and minelike targets. NATO Advanced Research Workshops have been held to

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Fig. 1 Vapor pressure versus temperature curves for a number of common explosives and related materials. The solid lines are the experimentally measured temperature ranges; the dashed lines are extrapolations

spawn new ideas and review progress to date. Many new small companies and/or consortia have been formed to investigate and/or exploit promising new technologies. All of this activity is a direct consequence of the urgency of this detection problem. Nevertheless, a silver bullet has not been found, nor is there one on the horizon. A number of reviews have been hidden within larger papers [11–16], such as the treatise on canine detection by Harper et al. [11] and the preview of next generation detectors by Lareau [12]. Nambayah and Quickenden provided a quantitative comparison of a limited number of methods based on literature quoted detection limits [13]. There are, however, concerns with quoted detection limits because of the dearth of standard reference materials. This void is being filled with the availability of a NIST trace vapor calibrator [17], which will allow higher accuracy LOD determinations as well as cross-comparability of LOD obtained on different instruments. In addition, known very small amounts of solids can be produced using ink jet technology [18] and pneumatically assisted nebulization [19]. Nanometer sized RDX particles have been prepared using aerosol jet techniques [20]. Given a preparation protocol for accurate and reproducible size and density, these techniques could be useful to calibrate trace surface detection methods. A few larger studies involving multiple detection methods are being undertaken. These include the BIOSENS project in South East Europe [21] and a Swedish land mine and unexploded ordnance (UXO) detection project named “multi optical mine detection system” (MOMS) [22]. While these studies are not specifically aimed at IED detection, the results should provide valuable insight into what works in a variety of situations.

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Fig. 2 Trace vapor and surface explosives detection methods, arranged according to sampling protocol; includes commercial, in development, and conceptual methods

The remainder of this review is organized like Fig. 2, where the methods used to detect and identify explosives are arranged by the sampling protocol used. Surface sampling is divided into contact and non-contact methods. The non-contact methods are divided into those that have achieved or are capable of stand-off usage and those that can be classified as “near-field” where the sample actually has to pass through the instrument to be detected. Contact sampling is divided into swipe, in-place, and vaporization methods depending on whether swipes are used to sample a surface, a reagent is sprayed onto the surface, or some means is used to volatilize the material on a surface. Some methods have been demonstrated for more than one sampling protocol. The discussion below begins with sampling issues and solutions for both surfaces and vapors. The abbreviations and acronyms are defined in the glossary.

4 Sampling and Preconcentration Because vapor phase concentrations of most explosives are so low, sampling and preconcentration are necessary to achieve reasonable ROC (receiver operating characteristic) curves, which allow comparison of detection methods on an equal playing field, on the basis of their sensitivity and selectivity (specificity) [16]. ROC curves can be brought closer to the ideal by improving the magnitude of the signal change with/without analyte, or by reducing measurement error limits. One way to achieve the former is by increasing the amount of analyte

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by improved sampling or preconcentation. A large number of sampling and preconcentration methods have been previously reviewed [1,12].

4.1 Sampling Standard sampling methods have been used for explosives, especially for environmental studies [23]. Samples have been obtained using extraction methods such as supercritical fluid extraction [24], solid phase extraction [25], and solid-liquid extraction [26]. Swipes have been used to determine surface contamination, using new materials such as PTFE [27]. The efficacy and reproducibility of swipe sampling has been studied and various protocols compared [28]. For liquid samples, Lokhnauth and Snow have used a variation of solid phase extraction, termed stir-bar sorptive extraction (SBSE) wherein the SPE phase is attached to a stir bar, which provides improved solvent contact and better phase ratio (volume of solvent/volume of coating), and therefore improved recovery and lower detection limits [29]. At the border between sampling and preconcentration lies a new method involving SPE of large air volumes followed by SFE extraction and GC separation and detection of the explosive [30]. High vapor pressure explosives can be sampled directly in the air using bottles filled with extraction fluids and air sampling pumps [31]. A promising method to actively desorb explosives from surfaces has been demonstrated using a high power strobe lamp [32]. Another novel method, which has been used to detect landmines but not yet been demonstrated to desorb explosives, is time reversal acoustic focusing [33]. These and similar techniques take advantage of the stickiness of most explosives, which concentrates them on environmental or man made surfaces. The sudden large vapor concentration above the surface caused by flash desorption would allow detection using a variety of other methods. This area needs considerable new research and development.

4.2 Vapor Concentration Methods Various materials have been explored for their abilities to adsorb explosive molecules from the air. Cooks and his colleagues have a number of schemes, including single sided membrane introduction MS, where the same side of an absorbant membrane material is exposed to the air and then to the mass spectrometer, avoiding the analyte losses and time penalty seen in two sided MIMS [34]. Kannan et al., have performed a number of studies of polymer materials such as carbowax and poly(dimethyl siloxane) (PDMS) as adsorbing surfaces for SAW detection devices [35,36]. PDMS has also been used as a solid phase microextraction (SPME) preconcentration front end for ion mobility spectroscopy [37]. On the more esoteric, but perhaps very useable, side, Oxley and coworkers showed that hair, especially black hair, is a very good explosive vapor sorbent and can be used to indicate exposure and/or handling [38,39]. Finally, aircraft boarding passes have been used as sampling devices, with desorption performed using short wave infrared radiation and detection via MS techniques [40]. Many trace preconcentration methods have been developed for environmental monitoring of explosives contaminated sites [41]. These include the entire range discussed above, but most commonly SPE [42]. All of these methods can be exploited for preconcentration of trace explosives for detection and identification, but their utility depends on conduct of operations limitations.

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4.3 Calibration and Testing Protocols The US National Academies have called for improvements in sensitivity, selectivity, and comprehensiveness of explosives detection technologies, as well as verification standards. The lack of such standards has led researchers to provide their own, with more or less success and no chance of validation. For example, Thundat and colleagues utilized a calibrated vapor generator for PETN and RDX using ambient air flowing through a thermostated reservoir [43,44]. Holl has described several methods in use at the Bundeswehr Research Institute [45]. To help fill this standards void, Gillen and colleagues at NIST have developed a new method to provide calibrated trace vapor concentrations of explosives [17]. The method uses piezoelectric nozzles and a nonporous ceramic-coated platinum resistance temperature detector element to produce known calibrated concentrations of explosives in an air stream. By varying solution concentrations in the six-nozzle array, droplet injection rates, air flow, and the number of active nozzles, the system can provide continuous vapor concentrations over more than six orders of magnitude, from less than 1 pg/L to 1 µg/L. The availability of such a calibrated source will greatly aid future method comparisons and validation. In addition, some methods to produce known quantities of solids and surface deposits were discussed above in the recent reviews section [18–20]. 4.4 Trace Detection on Surfaces The extremely low vapor pressures of many explosives, coupled with plume transport studies that show many orders of magnitude variability in concentration over time at a single location, or spatial variability at a given time, have caused many researchers to direct their efforts towards detection of trace amounts of materials on surfaces. The reason is simple—a 5 µm diameter speck of solid RDX explosive has a mass of ∼ 90 pg and contains ∼ 300 billion molecules, or as many RDX molecules as in 1 L of equilibrium vapor pressure STP air. A 25th generation fingerprint may contain as much as 100 times this much material. However, as is the case for vapor detection, the presence of trace explosives on external surfaces where they can be found and measured depends on the care with which devices are packaged. For typical IEDs that have been stored for some time, the traces on the exterior may have evaporated away. On the other hand, other materials could contaminate the exterior from elsewhere in the storage location. Adding a fuse or trigger mechanism could also produce exterior contamination. Nevertheless, methods to measure explosives traces on surfaces, and to distinguish them from background clutter, are in great demand. Methods recently developed to detect trace deposits on surfaces include colorimetric chemistry using cymantrene embedded in a polymer and developed using UV radiation [46]. Desorption electrospray ionization (DESI) and desorption atmospheric pressure chemical ionization (DAPCI) have been used as sensitive and selective ionization methods for MS analysis of surface materials [47]. Large deposits were found to begin to saturate the DESI response (> 10 pg), but deposits as small as 1 pg/cm2 of RDX were detectable in positive ion mode. Cluster SIMS using C− ions has been used to analyze samples of explosives dispersed as 8 particles on silicon subtrates. The carbon cluster primary ion was found to greatly enhance characteristic SIMS signals from the explosives while inducing minimal degradation, allowing high doses rapid spatially resolved molecular information data acquisition [48]. Other spectroscopic methods have been recently used to detect trace surface deposits. These will be discussed below by method. The advantage of such spectroscopic methods is

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their ability to detect the trace explosives at a stand-off distance. Miziolek and colleagues have provided an in-depth look at recent progress in laser-based explosives detection methods [49].

5 Trace Vapor Detection 5.1 Trained Animals Canines continue to be the gold standard against which other explosives detection methods are judged. The actual odor chemical(s) that the dogs detect is still a subject of investigation, although there is significant recent progress [11,50]. In the meantime, other animals are under investigation. Honey bees have been trained to locate buried land mines and tracked using LIDAR methods [51], with application to stand-off detection. Moths have also been trained on explosives odors [52]. A significant body of work has been performed assessing the capabilities of rats [53–55], which shows detection similar to dogs, smaller size, lower breeding and housing costs, the possibility of parallel and automated training, and shorter training times [54,55]. 5.2 Separations Methods A method to detect TATP in ambient air using reversed phase high performance liquid chromatography (HPLC) with post column UV irradiation and electrochemical detection was developed and shown to have a detection limit of 550 ng/L air (sampling 12 L volume) [31]. A similar HPLC, UV photo-assisted electrochemical detection (HPLC-UV-PAED) technique was used for the detection of trace explosives in ground water and soil extracts [42]. The vapor pressure of TATP versus temperature was studied using GC/ECD [3]. Holmgren et al., have determined nitroaromatics, cyclic nitramines, and nitrate esters using LC-MS with a porous graphitic carbon (Hypercarb) column [56] with improved detection limits ranging from 0.5 to 41.2 ng. The HPLC methods still suffer from long analysis times (10s of min). Holmgren et al., used a short column (100 mm) and reduced these times to 3–17 min, however with loss of separation of some analytes [5]. Direct vapor phase analysis has been performed using gas chromatography. The latest improvements have come from short high efficiency columns, where the GC analysis only takes a few seconds. As an example, a 1-m long resistively heated capillary column was coupled to an uncoated solid-state crystal SAW detector [57]. Heating rates up to 20◦ C/s produced 10-s chromatograms with peak widths of a few ms. Buryakov uses a multicapillary gas chromatographic column (MCC) of 1000 longitudinal parallel uniform capillaries each coated with a stationary liquid phase [58]. Gruznov et al., use the same column type [59]. The MCC achieves an efficiency of about 15 000 theoretical plates/m. The MicroChemLabTM from Sandia National Lab [60,61] uses a 86 cm length spiral GC column etched into silicon (Bosch deep reactive ion etching) 400 µm deep by 100 µm wide and capped with a Pyrex lid. The total chip area used is < 1.5 cm2 . The design permits rapid heating rates (6.5◦ C/s) at 3.8 W; faster with more heating power. The chips can be stacked if longer runs are needed. Detection has been demonstrated using arrays of SAW devices with different coatings, suspended membrane micro hotplates, or micron scale cylindrical ion traps fabricated from molded tungsten. Explosives vapors can be collected and preconcentrated (MicroHoundTM ) and then desorbed into the MicroChemLabTM for analysis [62,63].

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Other lab-on-a-chip microfluidic efforts for HE detection include an integrated collection, preconcentration, and analysis system that utilizes electrowetting transport with analyte concentrations measured by microscale colorimetric techniques [64] and a fully automated version of this device shown capable of detecting TNT at the 12 µg/mL level in liquids [65], but the analysis takes 5 min. There have also been a number of new microchip electrophoresis devices, which have been recently reviewed by Pumera [66]. 5.3 Nanotechnology The border between MEMS devices and nanotechnology for detection devices is becoming increasingly blurred. Larsson and colleagues demonstrated a novel biochip for TNT detection using self-assembled monolayers (SAMs) of hydroxyl-terminated oligo(ethylene glycol)-thiols containing three different TNT analogues in different proportions and either surface plasmon resonance (SPR) or quartz crystal microbalance (QCM) transduction [67]. The detection limit was found to fall in the 1–10 ng/mL range in liquids, and the analysis times were 100–400 s. An alternative transduction scheme uses field effect transistors (FET) based on organic materials. The binding of nitroaromatic molecules to the thin organic films, which form the transistor channel, increases the film conductivity and thereby the transistor electrical characteristic, which amplifies the signal [68]. Patel and collaborators use functionalized polymer-coated micromachined capacitors to measure the dielectric permittivity of selectively-adsorbed analytes [69]. An optical fiber based explosives detector based on defect free zeolite films grown on the straight cut face of a standard communication optical fiber [70] utilizes changes in the sensor reflectivity on exposure to materials of the size and shape dictated by the zeolite pores to measure concentration. Selectivity has not yet been demonstrated. Analysis times were ca. 200 s. Nanosized molybdenum hydrogen bronze reacts with TATP causing a color change from dark blue to yellow. The color change can be used in neutralization titrations as well as for detection [71]. Quantum dots have also been used to detect TNT [72]. The dot (CdSe with a ZnS shell) fluorescence was excited off resonance using 100 fs pulses at 400 nm (frequency doubled Ti:sapphire). The fluorescence was observed to shift and quench with added TNT (in solution at 1 ppm). Nanocrystalline porous Si films have been used to detect adsorbed nitroaromatic compounds at the ppb by volume level in flowing air via photoluminescence (PL) quenching [73]. Germanenko et al., took this idea further and measured the PL quenching and decay dynamics in Si nanocrystals [74]. Their results provided evidence for electron transfer from the Si conduction band to the LU orbitals of the quenchers, and support a PL model involving surface states in quantum confined Si nanomaterials. Much work in this area remains to be done. 5.4 Microcantilevers Microcantilevers have a unique and extremely sensitive sensing mechanism through their bending. Their high surface to volume ratio allows surface analyte interactions to induce large surface forces. Restricting those forces to just one surface using selective coatings results in differential stresses that cause the cantilever to bend. Microcantilevers are true MEMS devices and hundreds of sensors can be accommodated on a single MEMS chip. The recent advances have been made in the technology used to detect the bending and avoid

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use of the typical laser bouncing off the microcantilever tip. Pinnaduwage and collaborators make use of commercial piezoresistive microcantilever arrays [75]. They coated one of the microcantilevers with a SAM of 4-mercaptobenzoic acid as a hydrogen bonding coating for the analytes. They achieved LOD in the low parts per trillion range, but not selectivity between analytes yet. They do rely on diffusion of the analyte to the detector and the sticking coefficient, so analysis times can be seconds to minutes. Nanoporous coatings have been demonstrated to produce micrometer scale bending responses in the presence of vapor phase TNT, 1-MNT, and 2,4-DNT [76]. They found a noise-limited detection limit for TNT of 520 parts per trillion (by volume; pptv). Li et al., utilized piezoresistive elements constructed from thin single-crystalline silicon and fully encapsulated by SiO2 and coated with a functionalized SAM to produce a portable microcantilever TNT detector [77]. They measured a detection limit near 20 pptv. Voiculescu and collaborators designed and fabricated a resonant microcantilever beam in CMOS technology with piezoresistive transduction [78]. Metalloporphyrins have been used as a sensing surface material for nitroaromatics in a quartz crystal microbalance sensor [79]. Several groups have adapted microcantilevers or other microscale thermal devices to detect explosives via deflagration or nanocalorimetry. Pinnaduwage and colleagues detected TNT deposited on a pulse-heated piezoresistive microcantilever via deflagration induced bending and resonance frequency shifting [80,81]. Explosive vapors were found to have responses different from interferents such as water or alcohol. They also used a similar device to measure the desorption characteristics of various explosives and common inerts [82]. Liu et al., are developing a nanocalorimetry system using a microreactor for application to rapid screening of energetic materials at low cost [83].

6 Ion Detection Methods 6.1 Mass Spectrometry Mass spectrometric methods continue to lead the field for selectivity and sensitivity. Some of the MS methods also achieve short (ca. 5 s) analysis times by reducing or eliminating sample preparation. Work has been aimed at new sample introduction methods, improved portability, and size and cost reductions. A method based on liquid chromatography electrospray ionization mass spectrometry in negative ion mode using organic acid adduct ion detection for quantitative HMX analysis was developed by Pan et al., [84]. They achieved an LOD of 0.78 pg for HMX in solution and a linear calibration curve from 0.5 to 50 µg/L. Cooks and colleagues have explored desorption electrospray ionization (DESI) mass spectrometry for detection and identification of RDX, HMX, TNT, PETN, C-4, Semtex-H, and Detasheet on surfaces [47]. They demonstrated enhancement of the method when reactive additives are included in the spray solvent, which produce ionic adducts of the explosives. They then used the method to detect TATP on paper, brick and metal surfaces via alkali metal ion complexation and collision induced dissociation [85]. LODs were reported in the 1–50 ng range. Most recently, the use of the method at stand-off distances up to 3 m from the mass spectrometer has been reported [86]. The University of Puerto Rico at Mayaguez group has used TOF MS to measure the kinetic energy distributions of NO and NO2 UV photofragmentation (266 nm 100 femtosecond pulses at 500 Hz) products from TNT crystals, TNT vapor, and TNT in sand [87]. The differences observed could perhaps be utilized to detect and identify energetic materials. Martin et al., have applied single-particle aerosol mass spectrometry (SPAMS) to identification of micrometer-sized single explosive particles [88]. They used 7 ns pulses

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at 266 nm to vaporize and ionize the particles, and two reflectron-TOF mass analyzers in opposite directions to simultaneously detect positive and negative ions. Methods to increase the efficiency of sample introduction into the mass spectrometer are also being investigated. Gillen and his collaborators have explored the use of C− carbon 8 clusters in secondary ion mass spectrometry (SIMS) to reduce sample degradation [48]. Secondary electrospray ionization (SESI) has been used to detect adduct ions of RDX, NG, and PETN. Using a nonvolatile adduct-forming agent led to large improvements in detection limit, down to 5 µg/L for aqueous solutions of RDX [89]. Both femtosecond and nanosecond laser photoionization has been used to sensitively detect TATP using time-of-flight mass spectroscopy. A much larger fraction of parent and high mass fragments were obtained using the 130 fs pulses at 795 nm wavelength compared to 5 ns pulses at 266 nm and also to electron impact ionization [90]. This same group then explored the advantages of single-photon ionization for TOF-MS detection of nitrobenzenes and nitrotoluenes [91]. The required 118.2 nm photons were produced by focusing the 30 mJ third harmonic (355 nm) output of a 5 ns pulse length Nd:YAG laser into a gas cell containing Xe or Xe/Ar mixtures to achieve frequency tripling. The single photon ionization resulted in nearly complete parent ion plus parent-OH ion species, with very little fragmentation, giving LOD in the 10–50 ppbv range in air. The large resonance electron capture cross sections at low electron energies of nitrogen rich compounds has been taken advantage of using a tunable energy electron monochromator (TEEM) as the ionization source in GC/MS. The TEEM allows tuning of the ionization energy while monitoring electron capture resonances in real time, with resulting detection limits as low as 175 fg (TNT) [92]. Stamboli et al., have used headspace-GC-ion trap MS to detect TATP. The LOD after optimizing all experimental parameters (especially for low thermal decomposition) was found to be 0.1 ng using injections of 1 mL of headspace gas [93]. 6.2 Ion Mobility Spectrometry IMS appears to be a mature technology with many commercial instruments in place for trace HE detection. Analysis is by vapor sampling or swipes, and takes less than 1 min in most cases. Denton et al., showed that these commercial instruments should be modified to detect positive as well as negative ions in order to permit efficient detection of peroxides [94]. They also found that the presence of toluene dramatically improved the detection limit for TNT to 187 µg/mL. Clowers et al., have developed an IMS system that improves the duty cycle to 50% by the use of Hadamard transform techniques [95]. The Hadamard transform method improved the signal to nosie ratio by factors of 2 to 10 without reducing the instrumental resolution. Sandia National Lab researchers have assembled and are testing RobohoundTM , a robotic platform with manipulator arm, chemical sampling, and a commercial IMS explosives detector [96]. Given the extensive numbers of IMS units presently in use, Wallis and collaborators have developed a field performance and maintenance protocol to help ensure their correct operation over a wide range of field conditions and extended usage times [97].

7 Vibrational Spectroscopic Methods Some of the greatest improvements in explosives detection during the past 3 years have been made in the area of vibrational spectroscopy. This statement is especially true if we include THz spectroscopy in this area. Several of these methods have been shown capable of detecting explosives on surfaces at stand-off distances, although much work remains to be done to

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improve selectivity and differentiation from matrix effects and background clutter. A large fraction of energetic materials contain -NO2 groups, whose vibrational signatures are very characteristic. As an aid to further application of vibrational spectroscopic detection methods, Beal and Brill have provided a comprehensive treatise on the behavior of the vibrational spectra of the -NO2 group in more than 50 energetic compounds [98]. They also note that the insensitivity of the scissor motion frequencies (842–846 cm−1 ) to differences in the local chemical environment could enable them to be used as detection tags. Similarly, the presence of nitro group out-of-plane deformation bands within the range 760–767 cm−1 could be used in a similar way. 7.1 Infrared Absorption Spectroscopy There have been a few advances during the past 3 years in gas phase absorption methods for detection of explosives vapors. Willer et al., showed how difference frequency generation using two narrow band tunable diode lasers can generate narrow band tunable mid infrared, and demonstrated detection of NO vapor from laser ablation of different trace amounts of explosives on surfaces [99]. They propose to distinguish between explosives via the time dependent NO production rate with ablation laser pulse energy, repetition rate, and wavelength. They also demonstrated a fiber coupled infrared attenuated total reflection (evanescent wave) device to detect trace atmospheric gases, which could be adapted for explosives vapor detection given suitable coating materials to adsorb the target molecules. Other advances in infrared detection of explosives on surfaces have been obtained by Hernandez-Rivera and colleagues. This group has detected explosives on surfaces using a fiber-coupled FTIR probe [100], studied the temperature dependence of the limit of detection of TNT on metallic surfaces using fiber optic couple FTIR [101], detected and classified explosives mixtures on surfaces using grazing angle FTIR [102], demonstrated laboratory scale stand-off detection of RDX and TNT on reflective surfaces [103], and characterized thermal inkjet produced surface deposits of TNT using grazing incidence FTIR [18]. They have also measured the infrared spectral changes of TNT, RDX, and PETN attached to clay and other environmental materials [104–106]. A new differential absorption LIDAR (DIAL) system based on mid-infrared tunable optical parametric oscillators pumped by compact Q switched lasers is intended to detect nitrocompounds such as TNT, DNT, MNT, and RDX [107]. An IR point sensor consisting of a solid state IR emitter, polymer sorbent on IR windows and a multispectral IR detector is under development [108]. 7.2 Raman Spectroscopy Raman spectroscopy has been demonstrated to be capable of stand-off detection of trace quantities of explosives on surfaces. Some advances in stand-off Raman were reviewed or presented at the GEORAMAN 2004 conference [109]. Sharma et al., have tested a portable stand-off Raman system based on a 35 mJ 20 Hz frequency doubled 532 nm Nd:YAG excitation laser, a 125 mm aperture Cassegrain Maksukov telescope (f/15), an f/2.2 spectrograph with 100 µm slits, and a gate intensified CCD detector. The Raman signal collected by the telescope was filtered from the Rayleigh scattered excitation using a notch filter. The filtered Raman signal was passed to the spectrometer either using optical fibers or by direct optical coupling. The direct optical coupling method was found to be about 10 times as efficient (for

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their optical parameters). They have used their system to detect mg quantities of HMX and TATB at a distance of 10 m [110]. Carter and collaborators extended this range to 50 m using a 200 mm aperture f/11 Schmidt-Cassegrain telescope and a higher power frequency doubled Nd:YAG laser (up to 140 mJ) [111]. Sharma et al., have demonstrated 100 m stand-off Raman using using direct optical coupling of the 125 mm aperture telescope to the spectrograph, but not on explosives samples [112]. All of these studies utilized signal-averaged spectra from multiple laser shots (several hundred) to achieve sufficient signal to noise to discern the explosives samples. This signal averaging implies data accumulation times of > 10–100 s with present technology for fingerprint sized samples at > 10 m stand-off. Carter et al., have also compared conventional grating based and acousto-optic tunable filter based spectrometers for stand-off Raman signal to noise and throughput [113]. In their study, the fiber optic coupling apparently limited the signal level, so that the larger AOTF aperture could not be adequately utilized. The AOTF system’s capability to provide Raman imaging at stand-off distances (by tuning the AOTF to the wavelength of an appropriate Raman resonance) was demonstrated using non-explosives. They also measured the optimum ICCD gate width for the AOTF based system, which was found to be 2 µs (1 µs widths were found to be problematic due to laser/ICCD gate pulse timing jitter). A major issue with the use of Raman spectroscopy to detect explosives is that of background clutter, not only in the form of fluorescence, but also from Raman signals from matrix materials or surfaces. Fluorescence interference can be reduced but not eliminated by moving to redder excitation wavelengths. This tactic suffers from dramatically decreased Raman cross sections (which vary as ν 4 in wavelength ranges without electronic excitations), and also run up against the limitation of silicon based detectors (whose efficiency drops dramatically above 1000 nm). Alternatives are to use InGaAs or other near-infrared detector materials and suffer the ν 4 penalty, or to move into the UV where both the ν 4 improvement and electronic resonance enhancements provide much larger Raman signal strength [114,115]. Lewis et al., have compared Raman spectra of a variety of energetic materials at three different excitation wavelengths: 785, 830 and 1064 nm [116]. For the 1064 nm excitation, anti-Stokes Raman spectra were recorded [117]. The signal to noise and Raman peak to fluorescence intensities were measured in each material at each excitation wavelength, with the conclusion that 830 nm excitation is the best compromise for a field-portable instrument. Their other conclusion, however, was that Fourier-transform Raman using 1064 nm excitation and a portable FT spectrometer is an exciting future possibility with the recent introduction of a < 20 kg commercial system. Fluorescence interferences have been reduced using both photobleaching and background subtraction methods [118,119]. Photobleaching uses extended illumination of the sample, which serves to bleach fluorescent compounds if trap states are available. Such a method will have difficulty in applications involving short illumination time stand-off detection. A variety of baseline removal (background subtraction) methods have been used. Noonan et al., measured the background fluorescence spectrum in separate experiments and subtracted that spectrum from the Raman plus fluorescence spectrum [118]. Alternatively, Hasegawa et al., utilized the photobleaching phenomenon coupled with principal component analysis to separate the Raman spectrum from the fluorescence background [120]. Ben-Amotz and coworkers demonstrated a method based on second-derivative variance minimization, but it requires a priori measurement of the background or interferences [121]. The baseline can be fit and removed manually by picking points and using spline segments or polynomials, as is done in much commercial spectroscopic software. The accuracy and precision of such manual baseline removal methods has been analyzed [122]. Blades and colleagues have provided a quantitative comparison of baseline removal methods,

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which they classify into methods requiring knowledge of the baseline b, blurring function p and the noise function n (e.g., maximum entropy method), methods requiring estimates of the baseline (e.g., neural networks, threshold based classification, signal removal methods, composite baseline method, spectral shift methods), methods requiring no explicit knowledge of b, p, or n (e.g., noise median method and first derivative method), and methods requiring information about frequency (e.g., Fourier transform method, wavelet transform method) [123]. Each method is compared regarding their strengths, weaknesses, and amenability to automation, which will be useful for future studies of stand-off Raman of energetic materials in a variety of matrices and on many different types of surfaces. A newer rolling circle filter method based on the difference of the radii of curvature of Raman lines and the background has been successfully demonstrated as long as the widths of the Raman lines are significantly less than the background bandwidth [124]. 7.3 Surface Enhanced Raman Scattering (SERS) Nobel metal nanoparticles or nanostructures have been observed to cause large enhancements of the Raman signals for adsorbed molecules through a combination of localized surface plasmon resonance and induced increases in molecular bond polarizability. There have been further applications of SERS to detection of vapor phase explosive molecules during the past 3 years. Spencer and his collaborators used electrochemically roughened gold and silver substrates to detect the vapor signature over a TNT based landmine, and have produced a fieldable SERS vapor sensor with volumetric flow rates up to 0.4 L/s, which has been successfully used to detect landmines [125]. Reproducible and robust SERS substrates continue to be sought [126], and a large number of methods to achieve this goal have been recently reviewed [127]. Enhancement of TNT Raman signals on non-noble metal materials is being investigated [128]. These SERS methods rely on accumulation of the analyte from the vapor onto the substrate, implying minute range analysis times when diffusion controlled, or faster if active air movement schemes are utilized. Smith et al., have continued to explore the capabilities of combining surface enhancement with resonance enhancement [129]. They have used surface enhanced resonance Raman scattering (SERRS) in combination with clever species-specific chemistry to detect and distinguish explosives in solution. Using azo dyes containing electron-donating moieties for efficient diazo coupling and strong silver complexing groups to attach the product molecule to the SERS substrate allowed detection of TNT at near nM concentrations. They have also incorporated the chemistry into a microfluidics device [130]. Similar SERRS technology was interrogated at stand-off distances up to 20 m using a commercial portable Raman system adapted with a telephoto lens (no specifics given) [129]. 7.4 Cavity Ring Down Spectroscopy Cavity enhanced spectrometric methods have been recently reviewed [131]. The Cadillac implementation of CRDS in the mid-infrared has still not been improved upon [132]. However, CRDS has recently been made field deployable by using fiber optics and evanescent wave devices [133]. Evanescent wave cavity ring down utilizes the change in quality (Q factor) of an optical cavity as a function of the absorbance by species within an evanescent field. This idea has been implemented in the visible and near IR using a Dove prism located in a high Q cavity formed by two high reflectivity mirrors, and diode lasers. The Dove prism allows the necessary linear light path while providing a total internal reflection for the

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evanescent wave. A similar concept could be implemented in the mid infrared, but no references were found. 7.5 Terahertz The recent spectacular growth of interest in the terahertz region of the spectrum (loosely defined as the frequency range from 0.1 to 10 THz; THz = 1012 Hz) derives from the capability of those frequencies to penetrate many non-metallic materials, allowing the possibility to locate hidden explosives. THz techniques can identify materials via the spectrum of their absorbance by low frequency molecular motions, similar to infrared absorption methods. The recent advances include fundamental studies of the THz spectra of target materials as well as packaging and other matrix materials, technological improvements, exploration of standoff detection, and combinations of THz with other methods. Terahertz methods are capable of rapid detection, i.e., < 1 s, depending on the distance and sample size. Fundamental spectroscopic studies of explosives include: the measurement of the RDX transmission spectrum for granular RDX [134]; the THz spectra of 4-nitrotoluene and 2,6dinitrotoluene with assignments aided by density functional theory calculations [135]; THz spectroscopy of granular RDX from 0.3 to 10 THz [136]; and the temperature dependent THz transmission spectra of oriented single crystals of RDX, HMX, and PETN [137]. Further work on single crystal materials remains to be done to extract the full polarization dependence. Time domain THz spectra of RDX, C-4, and ammonium nitrate were compared to long wave FTIR spectra by Huang et al. [138]. Imaging and spectroscopy of the plastic bonded explosives PBX 9501 (HMX based) and PBX 9502 (TATB) were measured near 1 THz by Funk et al., [139]. Burnett et al., have measured THz spectra of PETN, TNT, and RDX in polymer matrices as well as pure, and have presented the temperature dependent spectra of PETN and RDX [140]. They also measured THz spectra of two plastic bonded explosives, Semtex-H (a mixture of RDX, PETN, plasticizers and dyes) and SX2 (contains RDX), and compared THz spectra to Raman spectra of the materials. There are many more explosives whose THz spectra need to be measured. THz methods have been used to detect and identify explosives. Yamamoto et al., examined the ability of THz time domain spectroscopy (TDS) to detection C-4 in mail items [141]. THz pulsed spectroscopic imaging was used to detect and identify RDX using seven absorption features in the 0.3 to 10 THz range (5–120 cm−1 ) [142]. They also imaged the spatial distribution of RDX using reflection THz spectroscopic imaging. The ability to do THz imaging of specific substances using reflection techniques is crucial for eventual security screening applications. Other groups have also demonstrated THz reflection spectroscopy. Fitch et al., demonstrated this capability using specular reflection at 45◦ angle of incidence and a conventional TDS THz system [143]. The same group has presented results for THz diffuse reflection detection of RDX [144], and has addressed propagation and imaging of THz through porous or granular media [145]. Federici et al., discuss in depth estimates of the effective range, spatial resolution, and portability issues necessary for stand-off THz imaging and detection [146]. De Lucia presented a series of enabling developments—the ‘X’ factors–needed to firmly establish THz methodology in practice [147]. This basic information has led to the recent demonstrations of stand-off detection of explosives using THz methods. Zhong et al., reported the observation of RDX at distances up to 30 m using its 0.82 THz absorption peak, which lies between the 0.78 and 0.98 THz water absorption lines [148]. Finally, Lu and collaborators have proposed a THz biochip based on optoelectronic devices [149]. The device incorporates a membrane-type edge-coupled photonic THz transmitter on

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a thin glass substrate with integrated polyethylene channel for near field THz detection of biomolecules adsorbed in the polyethylene channel. The channel near-field micro detection concept was shown feasible using standard TDS THz methods.

8 Other Spectroscopic Methods A variety of ideas utilizing some other kind of optical spectroscopic transduction scheme has been thrown at the HE detection problem during the past 3 years. Several groups have adapted optical fiber transducers with explosives sensitive detection schemes. The silicalite MFI-type zeolite selective explosives absorber discussed above was grown on the end of an optical fiber, and the explosives were detected using changes in refractive index (or optical thickness) [67]. Tait and colleagues coated a fiber Bragg grating with polymer materials, and detected the expansion of the polymer when it adsorbs the target molecule via the changes in the Bragg grating period [150]. The grating period changes were detected using rapid tuning of a communication laser between preset locked wavelengths combined with transmission spectroscopy to locate the Bragg wavelength resonance. Cao and Zhang [151] have proposed a cataluminescence sensor array based on nanosized SrCO3 , γ -Al2 O3 and BaCO3 as catalysts for explosive gas mixtures. Cataluminescence is the emission of light during catalytic oxidation of molecules on a solid catalyst surface. Several groups have utilized differential reflection spectroscopy to detect trace explosives. Hummel et al. [152] measured the normalized difference between the reflectivities of two adjacent parts of a specimen in the UV/visible spectral region, and found distinctive peaks at 250 and 420 nm characteristic for TNT on the surface. The Hernandez-Rivera group [153] measured the UV/visible absorption/reflection spectra of TNT on various surfaces, at distances up to 27 feet. UV absorption methods have been used to detect photofragments and pyrolysis fragments from nitro functionalities. Beauchamp and Hodyss [154] used UV detection of NO produced in thermal decomposition of gas chromatography effluent for the sensitive detection of nitrobenzene (25 ng) and of 2,4-dinitrotoluene (50 ng). Cabalo and Sausa used a 248 nm laser to photofragment target residues on a substrate, and then a 226 nm laser to photoionize and detect the resulting NO fragment by resonance-enhanced multiphoton ionization (REMPI) [155]. 8.1 Fluorescent Polymers Swager and his collaborators have continued to advance the ability of semiconductive organic polymers to detect explosives vapors at extremely low levels [156]. Selectivity between explosives and interferents is being improved in the devices via a fundamental understanding of the energy transport mechanism along the poly(arylene ethynylene) backbone and its response to polymer structure, assembly architecture, and receptor characteristics [157]. Fisher et al., at Nomadics have continued to provide novel packaging and testing of the Fido sensor based on Swager’s fluorescent polymers. They have recently reported successful detection of simulated vehicle-borne IED targets using both vapor and swipe sampling [158]. Cross-reactive chemical sensors using fluorescent polymers with both narrow and broad specificity are being utilized in an artificial olfactory system for land mine detection [159]. These devices rely on the transport of trace vapor, but active air movement can produce detection within a few seconds. Other groups have jumped into the fray. Lamarque and collaborators have developed a polysiloxane based refractive index sensor, as well as a fluorescence quenching polymer

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(N-(2,5-ditertiobutylphenyl)-1,8-naphthalimide functionalized polystyrene) with 60% quantum yield, whose fluorescence intensity dropped by 45% after a 1 min exposure to DNT vapor [160]. Three different light emitting conjugated polymers (two poly phenylene vinylenes and a poly diphenylacetylene) showed strong fluorescence quenching when exposed to TNT and DNT vapor [161]. Toal and Trogler investigated luminescent polymers as well as resistive sensing using polymer-coated carbon black particles and luminescent polymetalloles for sensing explosives in aqueous solution [162].

9 Immunochemical Sensors Different transduction schemes have been investigated over the past 3 years to enhance the ability of immunochemical processes to detect explosives. The Shankaran and Matsumoto groups at Kyushu University have utilized surface plasmon resonance transduction and indirect competitive immunoreactions to detect TNT vapor with excellent detection limit (60 pptv) and large dynamic range (to 1000 ppbv) [163]. The method utilizes competitive inhibition via two polyclonal antibodies, one prepared from goat [164] and the other from rabbit [165]. Bowen et al., have also used SPR transduction to detect TNT in the gas phase at ppbv concentrations via a monoclonal antibody covalently bound to a SAM attached to a thin gold film [166]. Shriver-Lake and colleagues have developed a continuous flow displacement immunoassay biosensor for TNT and RDX, and have used it to detect sub-ppb levels of explosives in water samples or soil extracts in less than 5 min [167]. They extended similar ideas to produce an array biosensor for simultaneous multi-analyte detection [168]. The array is based on a planar waveguide patterned with small sensing regions and end-illuminated by a 635 nm diode laser via a line generator. The emitted fluorescence is imaged onto a cooled CCD camera, which provides detection limits similar to those obtained from standard ELISA. Charles et al., described a reversed-displacement immunosensor for TNT using a chemically modified glass capillary [169]. The device achieved a detection limit of 0.25 µg/L for TNT in seawater with analysis time under 5 min. A portable flow-injection immunosensor utilizing chemiluminescence inversely proportional to the analyte concentration has been shown capable of detecting below 0.1 µg/L TNT in the laboratory with analysis times under 10 min [170]. The important factors in this instrument are the competition between the analyte and the enzyme-tracer, the luminescence background signal, and the flow pattern inside the chip. Lee et al., used a set of antibody coated SAW resonators inside a flow cell to detect TNT and RDX vapors from plastic explosives [171]. A non-specific reference sensor was employed to minimize environmental effects and interferences. Finally, proximity-induced fluorescence resonance energy transfer of antibody labeled quantum dots was used to detect TNT in aqueous environments [172].

10 Electrochemical Sensors The wide variety of electrochemical processes have continued to be refined for lower explosives detection limits and better selectivity. As is the case for many of the other methods, new materials have been explored and exploited. Zhang et al., have investigated cathodic voltammetry using mesoporous SiO2 (MCM-41) modified glassy carbon electrodes, which showed nanomolar detection limits for TNT, TNB, DNT, and DNB [173]. The improved sensitivity

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over previously investigated electrode materials was attributed to the strong adsorption of nitroaromatic compounds by MCM-41 and the large electrode working surface area. An electrochemically preanodized screen-printed carbon electrode was found to sharpen the reduction peak and thereby improve substituent selectivity [174]. The analysis can be done in a single-run measurement simply by measuring the peak current ratio between analytes and an internal standard. Hrapovic and collaborators formed nanocomposites of metal nanoparticles together with carbon nanotubes (CNT) solubilized in Nafion for cyclic voltammetric detection of TNT and other nitroaromatics [175]. The best combination was a modified glassy carbon electrode (GCE) containing Cu nanoparticles and single walled CNT, with a reproducible TNT detection limit of 1 ppb and 3 order of magnitude linear dynamic range. Wang et al., have used GCE modified with multi-wall CNT to detect TNT at the sub-µg/L level by adsorptive stripping voltammetry with 10 min deposition times [176]. Masunaga et al., utilized a surface-polarization controlling method to measure electrochemical impedance, as well as an anthracene treated electrode surface, to improve the selectivity between aromatic nitro compounds [177]. The Wang group at NMSU has made several advances in electrochemical detection of explosives. Wang has reviewed several types of microchip devices from his group and others [178]. They have fabricated an amperometric detection system from a CE microchip and a disc detection electrode in a Plexiglas holder [179]. The holder facilitates the precise 3-D alignment between the CE channel outlet and the detection electrode without a 3-D manipulator. The system was tested using a mixture of four nitroaromatic pollutants, and the optimized separation time was < 2 min with limits of detection down to 12 ppb. They have also produced a remote underwater electrochemical sensing system to detect TNT in aqueous environments [180]. The system utilizes a carbon fiber working electrode and square wave voltammetry, mounted on a remotely operated surface vehicle with vision capabilities and wireless communication. Finally, they have developed a voltammetric method to detect the explosive taggant DMNB (2,3-dimethyl-2,3-dinitrobutane) with 60 µg/L detection limit [181].

11 Conclusions There have been many improvements on nearly every analytical methodology to prove their abilities to detect explosive traces and vapors at the lowest possible detection limits, at the furthest possible stand-off distance, and in the shortest possible time. There are still many areas where great improvements are possible, and which will be necessary to provide the detection tools adequate for the task as well as for routine field use. For example, to date only LIBS and Raman have been demonstrated to detect fingerprint size explosives samples at significant stand-off distances (> 10 m). The fluorescence problem for stand-off Raman detection of explosives on real-world surfaces is being attacked using deep UV excitation as well as time-gating techniques, but significant signal accumulation times (ca. 100 s) are required for very small mass samples. LIBS has been demonstrated able to detect fingerprint sized explosives residues on a car door at 30 m distance, with ca. 1 s data accumulation times. Terahertz methods have achieved great advances in the past few years, and show some promise for stand-off detection of surface contamination. A large amount of basic THz spectroscopy work is still needed on these materials. However, these and other surface contamination sensors rely on the sloppiness of illicit weapon manufacturers. Fluorescence amplifying polymer based explosive sensors have been widely deployed and tested, and as new sensing materials are developed for a greater variety of explosives, the

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method will continue to improve. All trace vapor point sensors are subject to large concentration excursions due to turbulent atmospheric flows from the source. Signal integration times of a few seconds may help smooth out some of these excursions. Also, instrument cost reductions could allow such devices to be distributed or to provide guidance along concentration gradients to the source. Mass spectroscopic methods show great promise, but are still held back from routine portable application by their size and cost. Ion mobility spectrometry appears to be mature, but great advances have been made recently in dual tube technology for both positive and negative ion detection, miniaturization, and data analysis. The methods reviewed here have shown recent improvements in vapor detection limits, selectivity, robustness for field application, and stand-off distance. They all still rely on the uniqueness of some kind of molecular signature for their selectivity, and the vapor detection methods continue to be plagued by the low volatility of many of the target analytes. Some schemes have recently been introduced to volatilize explosives attached to surfaces, locally increasing the vapor concentration and thereby their detection. There are large gains available in this area. Finally, explosives are unique molecules in one sense—they store considerable energy and release it “on command.” That unique attribute has not yet been exhaustively explored for detection.
Acknowledgements This work was performed under the auspices of the US Department of Energy National Nuclear Security Administration. The author thanks Jim Koster, Scott Kinkead, and David Robbins for support.

Glossary Explosives AN ANFO AP C-4 DADP DATB Detasheet DMNB 2,4-DNT EGDN HMTD HMX HNS H2 O2 NC NG NM NQ PBX PBX-9501 PBX-9502 PE-4 PETN ammonium nitrate; NH4 NO3 composition of ammonium nitrate and fuel oil ammonium perchlorate; NH4 ClO4 composition of 91 % RDX plus waxes and oils diacetone diperoxide diamino trinitro benzene composition of PETN and NC with plasticizers 2,3-dimethyl-2,3-dinitrobutane (an explosives taggant) 2,4-dinitrotolune ethylene glycol dinitrate; nitroglycol hexamethylenetriperoxidediamine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; octagen hexanitrostilbene hydrogen peroxide nitrocellulose; gun cotton nitroglycerine; nitro; glyceryl trinitrate; RNG; trinitroglycerine nitromethane nitroguanidine plastic bonded explosive plastic bonded explosive with HMX plastic bonded explosive with TATB British Comp C: RDX with waxes and/or heavy oils pentaerythritol tetranitrate; 2,2-bis[(nitroxy)methyl]-1,3-propanediol, dinitrate

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Picric acid RDX Semtex-H TATB TATP Tetryl TNM TNT Other AOTF CARS CCD CE CMOS CNT CRDS DAPCI DESI DIAL ECD ELISA FAP FET FTIR GC GCE HPLC ICCD IED IMS LC LIBS LIDAR LOD LU MCC MEMS MIMS MS NATO Nd:YAG PAED PDMS PL PTFE QCM REMPI ROC

2,4,6-trinitrophenol hexahydro-1,3,5-trinitro-1,3,5-triazine; cyclonite; hexogen composition of RDX and PETN with heavy oils and rubbers triamino trinitro benzene triacetone triperoxide methyl-2,4,6-trinitrophenylnitramine tetranitromethane trinitrotoluene acousto-optic tunable filter coherent anti-Stokes Raman scattering charge coupled device capillary electrophoresis complementary metal oxide semiconductor carbon nanotube cavity ring down spectroscopy desorption atmospheric pressure chemical ionization desorption electrospray ionization differential absorption LIDAR electron capture detector enzyme linked immunosorbent assay fluorescence amplifying polymers field effect transistor Fourier transform infrared gas chromatography glassy carbon electrode high performance liquid chromatography intensified CCD improvised explosive device ion mobility spectrometry liquid chromatography laser induced breakdown spectroscopy light detection and ranging limit of detection lowest unoccupied (molecular orbital) multicapillary column microelectromechanical systems membrane introduction MS mass spectrometry North Atlantic Treaty Organization neodymium doped yttrium aluminum garnet photo-assisted electrochemical detection poly(dimethyl siloxane) photoluminescence poly(tetrafluoroethylene) quartz crystal microbalance resonance enhanced multiphoton ionization receiver operating characteristic

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SAM SAW SBSE SERS SERRS SESI SFE SIMS SLE SPE SPIE SPME SPR STP TDS TEEM TOF-MS UV UXO

self assembled monolayer surface acoustic wave stir-bar sorption extraction surface enhanced Raman scattering surface enhanced resonance Raman scattering secondary electrospray ionization supercritical fluid extraction secondary ion MS solid-liquid extraction solid phase extraction the International Society for Optical Engineering solid phase microextraction surface plasmon resonance standard temperature and pressure time domain spectroscopy tunable energy electron monochromator time of flight mass spectrometry ultraviolet unexploded ordinance

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Relevant Conference References A. J. Sedlacek III, R. Colton, & T. Vo-Dinh (Eds.) Chemical and biological point sensors for homeland defense. In Proceedings of SPIE Vol 5269. SPIE, Bellingham, WA. Edward M. Carapezza (Ed.), Sensors, and command, control, communications, and intelligence (C3I) technologies for homeland security and homeland defense III. In Proceedings of the SPIE Vol. 5403. SPIE, Belllingham, WA.

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R. Jennifer Hwu, & Dwight L. Woolard (Ed.), Terahertz for military and security applications II. In Proceedings of the SPIE Vol. 5411. SPIE, Belllingham, WA. Russell S. Harmon, J. Thomas Broach, & John H. Holloway, Jr., (Ed.), Detection and remediation technologies for mines and minelike targets IX. In Proceedings of the SPIE Vol. 5415. SPIE, Belllingham, WA. Edward M. Carapezza (Ed.), Sensors, and command, control, communications, and intelligence (C3I) technologies for homeland security and homeland Defense IV. In Proceedings of the SPIE Vol. 5778. SPIE, Belllingham, WA. Michael J. DeWeert & Theodore T. Saito (Ed.), Photonics for port and harbor security. In Proceedings of the SPIE Vol. 5780. SPIE, Belllingham, WA. Theodore T. Saito (Ed.), Optics and photonics in global homeland security. In Proceedings of the SPIE Vol. 5781. SPIE, Belllingham, WA. R. Jennifer Hwu, Dwight L. Woolard, & Mark J. Rosker (Ed.), Terahertz for military and security applications III. In Proceedings of the SPIE Vol. 5790. SPIE, Belllingham, WA. Russell S. Harmon, J. Thomas Broach, & John H. Holloway, Jr. (Eds.), Detection and remediation technologies for mines and minelike targets X. In Proceedings of the SPIE Vol. 5794. SPIE, Belllingham, WA. J. C. Carrano, A. Zukauskas, A. W. Vere, J. G. Grote, & F. Kajzar (Eds.), Optically based biological and chemical sensing, and optically based materials for defense. In Proceedings of SPIE Vol. 5990. SPIE, Bellingham, WA. James O. Jensen, & Jean-Marc Thériault (Eds.), Chemical and biological standoff detection III. In Proceedings of the SPIE Vol. 5995. SPIE, Belllingham, WA. Edward M. Carapezza (Ed.), Sensors, and command, control, communications, and intelligence (C3I) technologies for homeland security and homeland defense V. In Proceedings of the SPIE Vol. 6201. SPIE, Belllingham, WA. Theodore T. Saito & Daniel Lehrfeld (Eds.), Optics and photonics in global homeland security II. In Proceedings of the SPIE Vol. 6203. SPIE, Belllingham, WA. Michael J. DeWeert, Theodore T. Saito, & Harry L. Guthmuller (Eds.), Photonics for port and harbor security II. In Proceedings of the SPIE Vol. 6204. SPIE, Belllingham, WA. Dwight L. Woolard, R. Jennifer Hwu, Mark J. Rosker, & James O. Jensen (Eds.), Terahertz for military and security applications IV. In Proceedings of the SPIE Vol. 6212. SPIE, Belllingham, WA. J. Thomas Broach, Russell S. Harmon, John H. Holloway Jr. (Eds.), Detection and remediation technologies for mines and minelike targets XI. In Proceedings of the SPIE Vol. 6217. SPIE, Belllingham, WA.

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Colin Lewis, & Gary P. Owen (Eds.), Optics and photonics for counterterrorism and crime fighting II. In Proceedings of the SPIE Vol. 6402. SPIE, Belllingham, WA. M. Krausa, & A. A. Reznev (Eds.), Proceedings of the NATO advanced research workshop on vapour and trace detection of explosives for anti-terrorism purposes, NATO Science Series II. Mathematics, physics and Chemistry – V. 167. Dordrecht: Kluwer J. W. Gardner, & J. Yinon, (Eds.), Proceedings of the NATO advanced research workshop on electronic noses and sensors for the detection of explosives, NATO science series II. mathematics, physics and chemistry – V. 159. Dordrecht: Kluwer H. Schubert, & A. Kuznetsov, (Eds.), Proceedings of the NATO advanced workshop on detection and disposal of improvised explosives, NATO security through science series – B: Physics and biophysics. Dordrecht: Springer

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