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

ORIGINAL PAPER

Recent Advances in Trace Explosives DetectionInstrumentation

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 detectionof explosives in the past 3 years. This is especially true for methods that can be used ata stand off distance, driven by the frightening increase in the use of improvised explosivedevices in both suicide and road side bombings. This review attempts to outline and enumer-ate these recent developments, with details about the improvements made as well as wherefurther improvements might come.

Keywords Trace detection · Trace analysis · Explosives · Trace analytical instrumentation ·Reviews

1 Introduction

The inventiveness and creativity of those that would do the civilized world harm are seem-ingly limitless. This fact has been true throughout history; today is no exception. Whilecivilized people might have difficulty understanding their enemies’ motivation, they can andmust use their own creativity to proactively conceive adequate defenses. The most recentalarming increase in number and violence of terrorist bombings has made the task of stand-off detection of improvised explosive devices extremely urgent. Yet, because of the varietyof 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 andmethodology applicable to trace, vapor, and stand-off explosives detection. It is also intendedto compare current capabilities to what is necessary for field use. As was the case with myearlier review [1], the focus here will be on results published in the archival scientific lit-erature, via both peer reviewed journals and proceedings volumes (and also some NationalLaboratory reports), rather than vendor information.

D. S. Moore (B)Shock and Detonation Physics Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USAe-mail: moored@lanl.gov

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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 gaseousproducts so that the reaction produces large changes in pressure. These attributes are alsocharacteristic of fuels and oxidizers; what differentiates explosives is their ability to not reactuntil initiated by shock or heat applied to a small volume, which expands rapidly throughthe material. A rapid decomposition (deflagration) without formation of a shock wave ischaracteristic of a low explosive. Lack of shock formation is ideal (even critical) for use ofenergetic materials as propellants. High explosives support a leading shock (the detonationfront) at velocities from 1 to 9 km/s. In certain circumstances, a high explosive only reactsat low order. Such circumstances might include small diameter charges (below the failurediameter) or low density. The sensitivity to impact, friction, and heat differentiates primaryfrom secondary explosives. While a typical high explosive has fuel and oxidizer in the samemolecule, other materials also yield large amounts of energy and gas in a very short time andhave therefore been exploited by terrorists. Even primary explosives (those whose initiationcharacteristics 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 peroxideexplosives [2].

The number and kinds of explosive materials utilized recently has necessitated more basicstudies on their properties, including those properties relevant to their detection. In particu-lar, a number of new studies of vapor pressure have been published [2–6]. These have beenincorporated into Fig. 1, revised and updated from Figure 1 of Reference 1. Some commonexplosives missing from the earlier figure have been added (NM, H2O2, TATB, DATB, NG,TNM) [3–6]. Fortunately, most of the newly utilized materials have significant vapor pres-sures, reducing the difficulty of vapor phase detection. On the other hand, traces of explosivematerials on the exterior of packaging disappear more quickly for higher vapor pressurematerials [7,8], so that methods relying on swipes or surface treatments may have a timedependence not seen for lower vapor pressure materials. Studies of other properties specificto detection methods are discussed in each section below.

There have been a number of studies of source term dynamics and vapor transport. Thetransport 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, caus-ing large fluctuations in concentration with space and time. In addition, packaging plays anextremely important role in concealing explosives [1]. Information on common methods usedto 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 rele-vant to trace detection of explosives [see Conference References]. New terahertz technologyand methods were presented at SPIE conferences on terahertz for military and security appli-cations. The coupling of sensors with command, control, communications, and intelligencetechnologies was the subject of SPIE meetings in 2004, 2005, and 2006. Many overlappingtechnologies were discussed in the SPIE meetings on detection and remediation technolo-gies 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 consortiahave been formed to investigate and/or exploit promising new technologies. All of this activ-ity is a direct consequence of the urgency of this detection problem. Nevertheless, a silverbullet 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 treatiseon canine detection by Harper et al. [11] and the preview of next generation detectors byLareau [12]. Nambayah and Quickenden provided a quantitative comparison of a limitednumber 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 willallow higher accuracy LOD determinations as well as cross-comparability of LOD obtainedon different instruments. In addition, known very small amounts of solids can be producedusing ink jet technology [18] and pneumatically assisted nebulization [19]. Nanometer sizedRDX particles have been prepared using aerosol jet techniques [20]. Given a preparationprotocol for accurate and reproducible size and density, these techniques could be useful tocalibrate trace surface detection methods.

A few larger studies involving multiple detection methods are being undertaken. Theseinclude the BIOSENS project in South East Europe [21] and a Swedish land mine andunexploded ordnance (UXO) detection project named “multi optical mine detection system”(MOMS) [22]. While these studies are not specifically aimed at IED detection, the resultsshould 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 detectand identify explosives are arranged by the sampling protocol used. Surface sampling isdivided into contact and non-contact methods. The non-contact methods are divided intothose that have achieved or are capable of stand-off usage and those that can be classifiedas “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 onwhether swipes are used to sample a surface, a reagent is sprayed onto the surface, or somemeans is used to volatilize the material on a surface. Some methods have been demonstratedfor more than one sampling protocol. The discussion below begins with sampling issues andsolutions for both surfaces and vapors. The abbreviations and acronyms are defined in theglossary.

4 Sampling and Preconcentration

Because vapor phase concentrations of most explosives are so low, sampling and preconcen-tration 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 theirsensitivity and selectivity (specificity) [16]. ROC curves can be brought closer to the idealby improving the magnitude of the signal change with/without analyte, or by reducing mea-surement 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 preconcentrationmethods have been previously reviewed [1,12].

4.1 Sampling

Standard sampling methods have been used for explosives, especially for environmentalstudies [23]. Samples have been obtained using extraction methods such as supercriticalfluid extraction [24], solid phase extraction [25], and solid-liquid extraction [26]. Swipeshave 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 pro-tocols compared [28]. For liquid samples, Lokhnauth and Snow have used a variation ofsolid phase extraction, termed stir-bar sorptive extraction (SBSE) wherein the SPE phase isattached to a stir bar, which provides improved solvent contact and better phase ratio (vol-ume of solvent/volume of coating), and therefore improved recovery and lower detectionlimits [29].

At the border between sampling and preconcentration lies a new method involving SPEof large air volumes followed by SFE extraction and GC separation and detection of theexplosive [30]. High vapor pressure explosives can be sampled directly in the air usingbottles filled with extraction fluids and air sampling pumps [31].

A promising method to actively desorb explosives from surfaces has been demonstratedusing a high power strobe lamp [32]. Another novel method, which has been used to detectlandmines but not yet been demonstrated to desorb explosives, is time reversal acousticfocusing [33]. These and similar techniques take advantage of the stickiness of most explo-sives, which concentrates them on environmental or man made surfaces. The sudden largevapor concentration above the surface caused by flash desorption would allow detection usinga 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 fromthe air. Cooks and his colleagues have a number of schemes, including single sided mem-brane introduction MS, where the same side of an absorbant membrane material is exposedto the air and then to the mass spectrometer, avoiding the analyte losses and time penaltyseen in two sided MIMS [34]. Kannan et al., have performed a number of studies of polymermaterials such as carbowax and poly(dimethyl siloxane) (PDMS) as adsorbing surfaces forSAW detection devices [35,36]. PDMS has also been used as a solid phase microextrac-tion (SPME) preconcentration front end for ion mobility spectroscopy [37]. On the moreesoteric, but perhaps very useable, side, Oxley and coworkers showed that hair, especiallyblack hair, is a very good explosive vapor sorbent and can be used to indicate exposureand/or handling [38,39]. Finally, aircraft boarding passes have been used as sampling de-vices, with desorption performed using short wave infrared radiation and detection via MStechniques [40].

Many trace preconcentration methods have been developed for environmental monitoringof explosives contaminated sites [41]. These include the entire range discussed above, butmost commonly SPE [42]. All of these methods can be exploited for preconcentration of traceexplosives for detection and identification, but their utility depends on conduct of operationslimitations.

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4.3 Calibration and Testing Protocols

The US National Academies have called for improvements in sensitivity, selectivity, andcomprehensiveness 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 successand no chance of validation. For example, Thundat and colleagues utilized a calibrated vaporgenerator 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 newmethod to provide calibrated trace vapor concentrations of explosives [17]. The methoduses piezoelectric nozzles and a nonporous ceramic-coated platinum resistance temperaturedetector 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 concentrationsover more than six orders of magnitude, from less than 1 pg/L to 1µg/L. The availabilityof such a calibrated source will greatly aid future method comparisons and validation. Inaddition, some methods to produce known quantities of solids and surface deposits werediscussed 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 stud-ies that show many orders of magnitude variability in concentration over time at a singlelocation, or spatial variability at a given time, have caused many researchers to direct theirefforts towards detection of trace amounts of materials on surfaces. The reason is simple—a5 µm diameter speck of solid RDX explosive has a mass of ∼ 90 pg and contains ∼ 300 bil-lion 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. How-ever, as is the case for vapor detection, the presence of trace explosives on external surfaceswhere 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 haveevaporated away. On the other hand, other materials could contaminate the exterior fromelsewhere in the storage location. Adding a fuse or trigger mechanism could also produceexterior contamination. Nevertheless, methods to measure explosives traces on surfaces, andto distinguish them from background clutter, are in great demand.

Methods recently developed to detect trace deposits on surfaces include colorimetricchemistry using cymantrene embedded in a polymer and developed using UV radiation [46].Desorption electrospray ionization (DESI) and desorption atmospheric pressure chemicalionization (DAPCI) have been used as sensitive and selective ionization methods for MSanalysis of surface materials [47]. Large deposits were found to begin to saturate the DESIresponse (> 10 pg), but deposits as small as 1 pg/cm2 of RDX were detectable in positiveion mode.

Cluster SIMS using C−8 ions has been used to analyze samples of explosives dispersed as

particles on silicon subtrates. The carbon cluster primary ion was found to greatly enhancecharacteristic SIMS signals from the explosives while inducing minimal degradation, allow-ing 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 andcolleagues have provided an in-depth look at recent progress in laser-based explosivesdetection methods [49].

5 Trace Vapor Detection

5.1 Trained Animals

Canines continue to be the gold standard against which other explosives detection methodsare judged. The actual odor chemical(s) that the dogs detect is still a subject of investiga-tion, although there is significant recent progress [11,50]. In the meantime, other animals areunder investigation. Honey bees have been trained to locate buried land mines and trackedusing LIDAR methods [51], with application to stand-off detection. Moths have also beentrained on explosives odors [52]. A significant body of work has been performed assessingthe capabilities of rats [53–55], which shows detection similar to dogs, smaller size, lowerbreeding and housing costs, the possibility of parallel and automated training, and shortertraining times [54,55].

5.2 Separations Methods

A method to detect TATP in ambient air using reversed phase high performance liquid chro-matography (HPLC) with post column UV irradiation and electrochemical detection wasdeveloped 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) techniquewas used for the detection of trace explosives in ground water and soil extracts [42]. Thevapor pressure of TATP versus temperature was studied using GC/ECD [3]. Holmgren etal., have determined nitroaromatics, cyclic nitramines, and nitrate esters using LC-MS witha porous graphitic carbon (Hypercarb) column [56] with improved detection limits rangingfrom 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, howeverwith loss of separation of some analytes [5].

Direct vapor phase analysis has been performed using gas chromatography. The latestimprovements have come from short high efficiency columns, where the GC analysis onlytakes a few seconds. As an example, a 1-m long resistively heated capillary column wascoupled to an uncoated solid-state crystal SAW detector [57]. Heating rates up to 20◦C/sproduced 10-s chromatograms with peak widths of a few ms. Buryakov uses a multicapillarygas chromatographic column (MCC) of 1000 longitudinal parallel uniform capillaries eachcoated with a stationary liquid phase [58]. Gruznov et al., use the same column type [59]. TheMCC 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 Pyrexlid. 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, sus-pended membrane micro hotplates, or micron scale cylindrical ion traps fabricated frommolded 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 collec-tion, preconcentration, and analysis system that utilizes electrowetting transport with analyteconcentrations measured by microscale colorimetric techniques [64] and a fully automatedversion 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 electrophoresisdevices, which have been recently reviewed by Pumera [66].

5.3 Nanotechnology

The border between MEMS devices and nanotechnology for detection devices is becomingincreasingly blurred.

Larsson and colleagues demonstrated a novel biochip for TNT detection using self-assem-bled monolayers (SAMs) of hydroxyl-terminated oligo(ethylene glycol)-thiols containingthree different TNT analogues in different proportions and either surface plasmon resonance(SPR) or quartz crystal microbalance (QCM) transduction [67]. The detection limit wasfound to fall in the 1–10 ng/mL range in liquids, and the analysis times were 100–400 s. Analternative transduction scheme uses field effect transistors (FET) based on organic materials.The binding of nitroaromatic molecules to the thin organic films, which form the transistorchannel, increases the film conductivity and thereby the transistor electrical characteristic,which amplifies the signal [68]. Patel and collaborators use functionalized polymer-coatedmicromachined capacitors to measure the dielectric permittivity of selectively-adsorbedanalytes [69].

An optical fiber based explosives detector based on defect free zeolite films grown onthe straight cut face of a standard communication optical fiber [70] utilizes changes in thesensor reflectivity on exposure to materials of the size and shape dictated by the zeolite poresto measure concentration. Selectivity has not yet been demonstrated. Analysis times wereca. 200 s.

Nanosized molybdenum hydrogen bronze reacts with TATP causing a color change fromdark blue to yellow. The color change can be used in neutralization titrations as well as fordetection [71]. Quantum dots have also been used to detect TNT [72]. The dot (CdSe with aZnS shell) fluorescence was excited off resonance using 100 fs pulses at 400 nm (frequencydoubled 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 com-pounds 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 decaydynamics in Si nanocrystals [74]. Their results provided evidence for electron transfer fromthe Si conduction band to the LU orbitals of the quenchers, and support a PL model involv-ing surface states in quantum confined Si nanomaterials. Much work in this area remainsto be done.

5.4 Microcantilevers

Microcantilevers have a unique and extremely sensitive sensing mechanism through theirbending. Their high surface to volume ratio allows surface analyte interactions to inducelarge surface forces. Restricting those forces to just one surface using selective coatingsresults in differential stresses that cause the cantilever to bend. Microcantilevers are trueMEMS 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 collaboratorsmake use of commercial piezoresistive microcantilever arrays [75]. They coated one of themicrocantilevers with a SAM of 4-mercaptobenzoic acid as a hydrogen bonding coatingfor the analytes. They achieved LOD in the low parts per trillion range, but not selectivitybetween analytes yet. They do rely on diffusion of the analyte to the detector and the stickingcoefficient, so analysis times can be seconds to minutes.

Nanoporous coatings have been demonstrated to produce micrometer scale bendingresponses in the presence of vapor phase TNT, 1-MNT, and 2,4-DNT [76]. They found anoise-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 fullyencapsulated by SiO2 and coated with a functionalized SAM to produce a portable micro-cantilever TNT detector [77]. They measured a detection limit near 20 pptv. Voiculescu andcollaborators designed and fabricated a resonant microcantilever beam in CMOS technologywith piezoresistive transduction [78]. Metalloporphyrins have been used as a sensing surfacematerial for nitroaromatics in a quartz crystal microbalance sensor [79].

Several groups have adapted microcantilevers or other microscale thermal devices to detectexplosives via deflagration or nanocalorimetry. Pinnaduwage and colleagues detected TNTdeposited on a pulse-heated piezoresistive microcantilever via deflagration induced bendingand resonance frequency shifting [80,81]. Explosive vapors were found to have responsesdifferent from interferents such as water or alcohol. They also used a similar device to measurethe desorption characteristics of various explosives and common inerts [82]. Liu et al., aredeveloping a nanocalorimetry system using a microreactor for application to rapid screeningof 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 ofthe MS methods also achieve short (ca. 5 s) analysis times by reducing or eliminating samplepreparation. Work has been aimed at new sample introduction methods, improved portabil-ity, and size and cost reductions. A method based on liquid chromatography electrosprayionization mass spectrometry in negative ion mode using organic acid adduct ion detectionfor quantitative HMX analysis was developed by Pan et al., [84]. They achieved an LOD of0.78 pg for HMX in solution and a linear calibration curve from 0.5 to 50 µg/L. Cooks andcolleagues have explored desorption electrospray ionization (DESI) mass spectrometry fordetection and identification of RDX, HMX, TNT, PETN, C-4, Semtex-H, and Detasheet onsurfaces [47]. They demonstrated enhancement of the method when reactive additives areincluded in the spray solvent, which produce ionic adducts of the explosives. They then usedthe method to detect TATP on paper, brick and metal surfaces via alkali metal ion complexa-tion and collision induced dissociation [85]. LODs were reported in the 1–50 ng range. Mostrecently, the use of the method at stand-off distances up to 3 m from the mass spectrometerhas been reported [86]. The University of Puerto Rico at Mayaguez group has used TOF MSto measure the kinetic energy distributions of NO and NO2 UV photofragmentation (266 nm100 femtosecond pulses at 500 Hz) products from TNT crystals, TNT vapor, and TNT insand [87]. The differences observed could perhaps be utilized to detect and identify energeticmaterials. 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 inopposite directions to simultaneously detect positive and negative ions.

Methods to increase the efficiency of sample introduction into the mass spectrometer arealso being investigated. Gillen and his collaborators have explored the use of C−

8 carbonclusters in secondary ion mass spectrometry (SIMS) to reduce sample degradation [48]. Sec-ondary electrospray ionization (SESI) has been used to detect adduct ions of RDX, NG, andPETN. Using a nonvolatile adduct-forming agent led to large improvements in detectionlimit, down to 5 µg/L for aqueous solutions of RDX [89]. Both femtosecond and nanosec-ond laser photoionization has been used to sensitively detect TATP using time-of-flight massspectroscopy. A much larger fraction of parent and high mass fragments were obtained usingthe 130 fs pulses at 795 nm wavelength compared to 5 ns pulses at 266 nm and also to electronimpact ionization [90]. This same group then explored the advantages of single-photon ioni-zation for TOF-MS detection of nitrobenzenes and nitrotoluenes [91]. The required 118.2 nmphotons were produced by focusing the 30 mJ third harmonic (355 nm) output of a 5 ns pulselength Nd:YAG laser into a gas cell containing Xe or Xe/Ar mixtures to achieve frequencytripling. The single photon ionization resulted in nearly complete parent ion plus parent-OHion 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 nitro-gen rich compounds has been taken advantage of using a tunable energy electron mono-chromator (TEEM) as the ionization source in GC/MS. The TEEM allows tuning of theionization energy while monitoring electron capture resonances in real time, with resultingdetection limits as low as 175 fg (TNT) [92]. Stamboli et al., have used headspace-GC-ion trapMS to detect TATP. The LOD after optimizing all experimental parameters (especially forlow thermal decomposition) was found to be 0.1 ng using injections of 1 mL of headspacegas [93].

6.2 Ion Mobility Spectrometry

IMS appears to be a mature technology with many commercial instruments in place for traceHE detection. Analysis is by vapor sampling or swipes, and takes less than 1 min in mostcases. Denton et al., showed that these commercial instruments should be modified to detectpositive as well as negative ions in order to permit efficient detection of peroxides [94]. Theyalso found that the presence of toluene dramatically improved the detection limit for TNT to187 µg/mL. Clowers et al., have developed an IMS system that improves the duty cycle to50% by the use of Hadamard transform techniques [95]. The Hadamard transform methodimproved the signal to nosie ratio by factors of 2 to 10 without reducing the instrumentalresolution. Sandia National Lab researchers have assembled and are testing RobohoundTM,a robotic platform with manipulator arm, chemical sampling, and a commercial IMS explo-sives detector [96]. Given the extensive numbers of IMS units presently in use, Wallis andcollaborators have developed a field performance and maintenance protocol to help ensuretheir 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 beenmade in the area of vibrational spectroscopy. This statement is especially true if we includeTHz spectroscopy in this area. Several of these methods have been shown capable of detect-ing 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 largefraction of energetic materials contain -NO2 groups, whose vibrational signatures are verycharacteristic. As an aid to further application of vibrational spectroscopic detection meth-ods, Beal and Brill have provided a comprehensive treatise on the behavior of the vibrationalspectra of the -NO2 group in more than 50 energetic compounds [98]. They also note thatthe insensitivity of the scissor motion frequencies (842–846 cm−1) to differences in the localchemical environment could enable them to be used as detection tags. Similarly, the presenceof nitro group out-of-plane deformation bands within the range 760–767 cm−1 could be usedin a similar way.

7.1 Infrared Absorption Spectroscopy

There have been a few advances during the past 3 years in gas phase absorption methods fordetection of explosives vapors. Willer et al., showed how difference frequency generationusing 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 ofexplosives on surfaces [99]. They propose to distinguish between explosives via the timedependent NO production rate with ablation laser pulse energy, repetition rate, and wave-length. They also demonstrated a fiber coupled infrared attenuated total reflection (evanescentwave) device to detect trace atmospheric gases, which could be adapted for explosives vapordetection given suitable coating materials to adsorb the target molecules.

Other advances in infrared detection of explosives on surfaces have been obtained byHernandez-Rivera and colleagues. This group has detected explosives on surfaces using afiber-coupled FTIR probe [100], studied the temperature dependence of the limit of detec-tion of TNT on metallic surfaces using fiber optic couple FTIR [101], detected and classifiedexplosives mixtures on surfaces using grazing angle FTIR [102], demonstrated laboratoryscale stand-off detection of RDX and TNT on reflective surfaces [103], and characterizedthermal inkjet produced surface deposits of TNT using grazing incidence FTIR [18]. Theyhave also measured the infrared spectral changes of TNT, RDX, and PETN attached to clayand other environmental materials [104–106].

A new differential absorption LIDAR (DIAL) system based on mid-infrared tunableoptical parametric oscillators pumped by compact Q switched lasers is intended to detectnitrocompounds such as TNT, DNT, MNT, and RDX [107]. An IR point sensor consistingof a solid state IR emitter, polymer sorbent on IR windows and a multispectral IR detector isunder development [108].

7.2 Raman Spectroscopy

Raman spectroscopy has been demonstrated to be capable of stand-off detection of tracequantities of explosives on surfaces. Some advances in stand-off Raman were reviewed orpresented at the GEORAMAN 2004 conference [109]. Sharma et al., have tested a portablestand-off Raman system based on a 35 mJ 20 Hz frequency doubled 532 nm Nd:YAG excita-tion laser, a 125 mm aperture Cassegrain Maksukov telescope (f/15), an f/2.2 spectrographwith 100 µm slits, and a gate intensified CCD detector. The Raman signal collected by thetelescope was filtered from the Rayleigh scattered excitation using a notch filter. The filteredRaman signal was passed to the spectrometer either using optical fibers or by direct opticalcoupling. 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 andTATB at a distance of 10 m [110]. Carter and collaborators extended this range to 50 m usinga 200 mm aperture f/11 Schmidt-Cassegrain telescope and a higher power frequency doubledNd:YAG laser (up to 140 mJ) [111]. Sharma et al., have demonstrated 100 m stand-off Ramanusing using direct optical coupling of the 125 mm aperture telescope to the spectrograph, butnot on explosives samples [112]. All of these studies utilized signal-averaged spectra frommultiple laser shots (several hundred) to achieve sufficient signal to noise to discern theexplosives samples. This signal averaging implies data accumulation times of > 10–100 swith present technology for fingerprint sized samples at > 10 m stand-off.

Carter et al., have also compared conventional grating based and acousto-optic tunablefilter based spectrometers for stand-off Raman signal to noise and throughput [113]. In theirstudy, the fiber optic coupling apparently limited the signal level, so that the larger AOTFaperture could not be adequately utilized. The AOTF system’s capability to provide Ramanimaging at stand-off distances (by tuning the AOTF to the wavelength of an appropriateRaman resonance) was demonstrated using non-explosives. They also measured the opti-mum ICCD gate width for the AOTF based system, which was found to be 2 µs (1 µs widthswere 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 back-ground clutter, not only in the form of fluorescence, but also from Raman signals from matrixmaterials or surfaces. Fluorescence interference can be reduced but not eliminated by mov-ing to redder excitation wavelengths. This tactic suffers from dramatically decreased Ramancross sections (which vary as ν4 in wavelength ranges without electronic excitations), and alsorun up against the limitation of silicon based detectors (whose efficiency drops dramaticallyabove 1000 nm). Alternatives are to use InGaAs or other near-infrared detector materials andsuffer the ν4 penalty, or to move into the UV where both the ν4 improvement and electronicresonance 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 excitationwavelengths: 785, 830 and 1064 nm [116]. For the 1064 nm excitation, anti-Stokes Ramanspectra were recorded [117]. The signal to noise and Raman peak to fluorescence intensi-ties were measured in each material at each excitation wavelength, with the conclusion that830 nm excitation is the best compromise for a field-portable instrument. Their other conclu-sion, however, was that Fourier-transform Raman using 1064 nm excitation and a portableFT spectrometer is an exciting future possibility with the recent introduction of a < 20 kgcommercial system.

Fluorescence interferences have been reduced using both photobleaching and backgroundsubtraction methods [118,119]. Photobleaching uses extended illumination of the sample,which serves to bleach fluorescent compounds if trap states are available. Such a methodwill have difficulty in applications involving short illumination time stand-off detection. Avariety of baseline removal (background subtraction) methods have been used. Noonan etal., measured the background fluorescence spectrum in separate experiments and subtractedthat spectrum from the Raman plus fluorescence spectrum [118].

Alternatively, Hasegawa et al., utilized the photobleaching phenomenon coupled withprincipal component analysis to separate the Raman spectrum from the fluorescence back-ground [120]. Ben-Amotz and coworkers demonstrated a method based on second-derivativevariance minimization, but it requires a priori measurement of the background or interfer-ences [121]. The baseline can be fit and removed manually by picking points and usingspline segments or polynomials, as is done in much commercial spectroscopic software. Theaccuracy 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 pand the noise function n (e.g., maximum entropy method), methods requiring estimates ofthe baseline (e.g., neural networks, threshold based classification, signal removal methods,composite baseline method, spectral shift methods), methods requiring no explicit knowledgeof b, p, or n (e.g., noise median method and first derivative method), and methods requiringinformation about frequency (e.g., Fourier transform method, wavelet transform method)[123]. Each method is compared regarding their strengths, weaknesses, and amenability toautomation, which will be useful for future studies of stand-off Raman of energetic materialsin a variety of matrices and on many different types of surfaces. A newer rolling circle filtermethod based on the difference of the radii of curvature of Raman lines and the backgroundhas been successfully demonstrated as long as the widths of the Raman lines are significantlyless than the background bandwidth [124].

7.3 Surface Enhanced Raman Scattering (SERS)

Nobel metal nanoparticles or nanostructures have been observed to cause large enhance-ments of the Raman signals for adsorbed molecules through a combination of localizedsurface plasmon resonance and induced increases in molecular bond polarizability. Therehave been further applications of SERS to detection of vapor phase explosive molecules dur-ing the past 3 years. Spencer and his collaborators used electrochemically roughened goldand silver substrates to detect the vapor signature over a TNT based landmine, and haveproduced a fieldable SERS vapor sensor with volumetric flow rates up to 0.4 L/s, which hasbeen successfully used to detect landmines [125]. Reproducible and robust SERS substratescontinue to be sought [126], and a large number of methods to achieve this goal have beenrecently reviewed [127]. Enhancement of TNT Raman signals on non-noble metal materialsis being investigated [128]. These SERS methods rely on accumulation of the analyte fromthe 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 enhance-ment with resonance enhancement [129]. They have used surface enhanced resonance Ra-man scattering (SERRS) in combination with clever species-specific chemistry to detect anddistinguish explosives in solution. Using azo dyes containing electron-donating moieties forefficient diazo coupling and strong silver complexing groups to attach the product moleculeto the SERS substrate allowed detection of TNT at near nM concentrations. They have alsoincorporated the chemistry into a microfluidics device [130]. Similar SERRS technology wasinterrogated at stand-off distances up to 20 m using a commercial portable Raman systemadapted with a telephoto lens (no specifics given) [129].

7.4 Cavity Ring Down Spectroscopy

Cavity enhanced spectrometric methods have been recently reviewed [131]. The Cadillacimplementation of CRDS in the mid-infrared has still not been improved upon [132]. How-ever, CRDS has recently been made field deployable by using fiber optics and evanescentwave devices [133]. Evanescent wave cavity ring down utilizes the change in quality (Qfactor) of an optical cavity as a function of the absorbance by species within an evanescentfield. This idea has been implemented in the visible and near IR using a Dove prism lo-cated in a high Q cavity formed by two high reflectivity mirrors, and diode lasers. The Doveprism 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 noreferences were found.

7.5 Terahertz

The recent spectacular growth of interest in the terahertz region of the spectrum (looselydefined as the frequency range from 0.1 to 10 THz; THz = 1012 Hz) derives from the capa-bility of those frequencies to penetrate many non-metallic materials, allowing the possibilityto locate hidden explosives. THz techniques can identify materials via the spectrum of theirabsorbance by low frequency molecular motions, similar to infrared absorption methods. Therecent advances include fundamental studies of the THz spectra of target materials as wellas packaging and other matrix materials, technological improvements, exploration of stand-off detection, and combinations of THz with other methods. Terahertz methods are capableof rapid detection, i.e., < 1 s, depending on the distance and sample size.

Fundamental spectroscopic studies of explosives include: the measurement of the RDXtransmission spectrum for granular RDX [134]; the THz spectra of 4-nitrotoluene and 2,6-dinitrotoluene with assignments aided by density functional theory calculations [135]; THzspectroscopy of granular RDX from 0.3 to 10 THz [136]; and the temperature dependent THztransmission spectra of oriented single crystals of RDX, HMX, and PETN [137]. Further workon 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 waveFTIR spectra by Huang et al. [138]. Imaging and spectroscopy of the plastic bonded explo-sives PBX 9501 (HMX based) and PBX 9502 (TATB) were measured near 1 THz by Funket al., [139]. Burnett et al., have measured THz spectra of PETN, TNT, and RDX in polymermatrices as well as pure, and have presented the temperature dependent spectra of PETN andRDX [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 comparedTHz spectra to Raman spectra of the materials. There are many more explosives whose THzspectra need to be measured.

THz methods have been used to detect and identify explosives. Yamamoto et al., exam-ined 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 sevenabsorption features in the 0.3 to 10 THz range (5–120 cm−1) [142]. They also imaged thespatial distribution of RDX using reflection THz spectroscopic imaging. The ability to doTHz imaging of specific substances using reflection techniques is crucial for eventual securityscreening applications. Other groups have also demonstrated THz reflection spectroscopy.Fitch et al., demonstrated this capability using specular reflection at 45◦ angle of incidenceand a conventional TDS THz system [143]. The same group has presented results for THzdiffuse reflection detection of RDX [144], and has addressed propagation and imaging ofTHz through porous or granular media [145]. Federici et al., discuss in depth estimates ofthe effective range, spatial resolution, and portability issues necessary for stand-off THzimaging 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 explo-sives using THz methods. Zhong et al., reported the observation of RDX at distances up to30 m using its 0.82 THz absorption peak, which lies between the 0.78 and 0.98 THz waterabsorption 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 ofbiomolecules adsorbed in the polyethylene channel. The channel near-field micro detectionconcept was shown feasible using standard TDS THz methods.

8 Other Spectroscopic Methods

A variety of ideas utilizing some other kind of optical spectroscopic transduction schemehas been thrown at the HE detection problem during the past 3 years.

Several groups have adapted optical fiber transducers with explosives sensitive detectionschemes. The silicalite MFI-type zeolite selective explosives absorber discussed above wasgrown on the end of an optical fiber, and the explosives were detected using changes inrefractive index (or optical thickness) [67]. Tait and colleagues coated a fiber Bragg gratingwith polymer materials, and detected the expansion of the polymer when it adsorbs the targetmolecule via the changes in the Bragg grating period [150]. The grating period changes weredetected using rapid tuning of a communication laser between preset locked wavelengthscombined with transmission spectroscopy to locate the Bragg wavelength resonance.

Cao and Zhang [151] have proposed a cataluminescence sensor array based on nanosizedSrCO3, γ -Al2O3 and BaCO3 as catalysts for explosive gas mixtures. Cataluminescence isthe emission of light during catalytic oxidation of molecules on a solid catalyst surface.

Several groups have utilized differential reflection spectroscopy to detect trace explo-sives. Hummel et al. [152] measured the normalized difference between the reflectivities oftwo adjacent parts of a specimen in the UV/visible spectral region, and found distinctivepeaks 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, atdistances up to 27 feet.

UV absorption methods have been used to detect photofragments and pyrolysis fragmentsfrom nitro functionalities. Beauchamp and Hodyss [154] used UV detection of NO pro-duced in thermal decomposition of gas chromatography effluent for the sensitive detection ofnitrobenzene (25 ng) and of 2,4-dinitrotoluene (50 ng). Cabalo and Sausa used a 248 nm laserto photofragment target residues on a substrate, and then a 226 nm laser to photoionize and de-tect 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 organicpolymers to detect explosives vapors at extremely low levels [156]. Selectivity between explo-sives and interferents is being improved in the devices via a fundamental understanding of theenergy transport mechanism along the poly(arylene ethynylene) backbone and its responseto 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 basedon Swager’s fluorescent polymers. They have recently reported successful detection of sim-ulated vehicle-borne IED targets using both vapor and swipe sampling [158]. Cross-reactivechemical sensors using fluorescent polymers with both narrow and broad specificity are beingutilized in an artificial olfactory system for land mine detection [159]. These devices relyon the transport of trace vapor, but active air movement can produce detection within a fewseconds.

Other groups have jumped into the fray. Lamarque and collaborators have developeda 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% quan-tum yield, whose fluorescence intensity dropped by 45% after a 1 min exposure to DNTvapor [160]. Three different light emitting conjugated polymers (two poly phenylene vinyl-enes and a poly diphenylacetylene) showed strong fluorescence quenching when exposed toTNT and DNT vapor [161]. Toal and Trogler investigated luminescent polymers as well asresistive sensing using polymer-coated carbon black particles and luminescent polymetallolesfor sensing explosives in aqueous solution [162].

9 Immunochemical Sensors

Different transduction schemes have been investigated over the past 3 years to enhancethe ability of immunochemical processes to detect explosives. The Shankaran and Mat-sumoto groups at Kyushu University have utilized surface plasmon resonance transductionand indirect competitive immunoreactions to detect TNT vapor with excellent detectionlimit (60 pptv) and large dynamic range (to 1000 ppbv) [163]. The method utilizes com-petitive inhibition via two polyclonal antibodies, one prepared from goat [164] and theother from rabbit [165]. Bowen et al., have also used SPR transduction to detect TNTin the gas phase at ppbv concentrations via a monoclonal antibody covalently bound toa SAM attached to a thin gold film [166].

Shriver-Lake and colleagues have developed a continuous flow displacement immunoas-say biosensor for TNT and RDX, and have used it to detect sub-ppb levels of explosives inwater samples or soil extracts in less than 5 min [167]. They extended similar ideas to pro-duce an array biosensor for simultaneous multi-analyte detection [168]. The array is basedon a planar waveguide patterned with small sensing regions and end-illuminated by a 635 nmdiode laser via a line generator. The emitted fluorescence is imaged onto a cooled CCDcamera, which provides detection limits similar to those obtained from standard ELISA.Charles et al., described a reversed-displacement immunosensor for TNT using a chem-ically modified glass capillary [169]. The device achieved a detection limit of 0.25 µg/Lfor TNT in seawater with analysis time under 5 min. A portable flow-injection immuno-sensor utilizing chemiluminescence inversely proportional to the analyte concentration hasbeen shown capable of detecting below 0.1 µg/L TNT in the laboratory with analysis timesunder 10 min [170]. The important factors in this instrument are the competition between theanalyte and the enzyme-tracer, the luminescence background signal, and the flow patterninside the chip. Lee et al., used a set of antibody coated SAW resonators inside a flowcell to detect TNT and RDX vapors from plastic explosives [171]. A non-specific referencesensor was employed to minimize environmental effects and interferences. Finally, proxim-ity-induced fluorescence resonance energy transfer of antibody labeled quantum dots wasused to detect TNT in aqueous environments [172].

10 Electrochemical Sensors

The wide variety of electrochemical processes have continued to be refined for lower explo-sives detection limits and better selectivity. As is the case for many of the other methods, newmaterials have been explored and exploited. Zhang et al., have investigated cathodic voltam-metry using mesoporous SiO2 (MCM-41) modified glassy carbon electrodes, which showednanomolar 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 ofnitroaromatic compounds by MCM-41 and the large electrode working surface area. Anelectrochemically preanodized screen-printed carbon electrode was found to sharpen thereduction peak and thereby improve substituent selectivity [174]. The analysis can be donein a single-run measurement simply by measuring the peak current ratio between analytesand an internal standard. Hrapovic and collaborators formed nanocomposites of metal nano-particles together with carbon nanotubes (CNT) solubilized in Nafion for cyclic voltammetricdetection of TNT and other nitroaromatics [175]. The best combination was a modified glassycarbon electrode (GCE) containing Cu nanoparticles and single walled CNT, with a repro-ducible TNT detection limit of 1 ppb and 3 order of magnitude linear dynamic range. Wanget al., have used GCE modified with multi-wall CNT to detect TNT at the sub-µg/L level byadsorptive stripping voltammetry with 10 min deposition times [176].

Masunaga et al., utilized a surface-polarization controlling method to measure electro-chemical impedance, as well as an anthracene treated electrode surface, to improve theselectivity between aromatic nitro compounds [177].

The Wang group at NMSU has made several advances in electrochemical detection ofexplosives. Wang has reviewed several types of microchip devices from his group and oth-ers [178]. They have fabricated an amperometric detection system from a CE microchipand a disc detection electrode in a Plexiglas holder [179]. The holder facilitates the precise3-D alignment between the CE channel outlet and the detection electrode without a 3-Dmanipulator. The system was tested using a mixture of four nitroaromatic pollutants, and theoptimized separation time was < 2 min with limits of detection down to 12 ppb. They havealso produced a remote underwater electrochemical sensing system to detect TNT in aque-ous environments [180]. The system utilizes a carbon fiber working electrode and squarewave voltammetry, mounted on a remotely operated surface vehicle with vision capabil-ities and wireless communication. Finally, they have developed a voltammetric method todetect the explosive taggant DMNB (2,3-dimethyl-2,3-dinitrobutane) with 60 µg/L detectionlimit [181].

11 Conclusions

There have been many improvements on nearly every analytical methodology to prove theirabilities to detect explosive traces and vapors at the lowest possible detection limits, at thefurthest possible stand-off distance, and in the shortest possible time. There are still manyareas where great improvements are possible, and which will be necessary to provide thedetection 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 fingerprintsize explosives samples at significant stand-off distances (> 10 m). The fluorescence problemfor stand-off Raman detection of explosives on real-world surfaces is being attacked usingdeep UV excitation as well as time-gating techniques, but significant signal accumulationtimes (ca. 100 s) are required for very small mass samples. LIBS has been demonstrated ableto detect fingerprint sized explosives residues on a car door at 30 m distance, with ca. 1 s dataaccumulation 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 ofbasic THz spectroscopy work is still needed on these materials. However, these and othersurface contamination sensors rely on the sloppiness of illicit weapon manufacturers.

Fluorescence amplifying polymer based explosive sensors have been widely deployed andtested, 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 concentra-tion excursions due to turbulent atmospheric flows from the source. Signal integration timesof a few seconds may help smooth out some of these excursions. Also, instrument cost reduc-tions could allow such devices to be distributed or to provide guidance along concentrationgradients to the source.

Mass spectroscopic methods show great promise, but are still held back from routineportable 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 andnegative 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 theuniqueness of some kind of molecular signature for their selectivity, and the vapor detectionmethods continue to be plagued by the low volatility of many of the target analytes. Someschemes have recently been introduced to volatilize explosives attached to surfaces, locallyincreasing the vapor concentration and thereby their detection. There are large gains availablein this area. Finally, explosives are unique molecules in one sense—they store considerableenergy and release it “on command.” That unique attribute has not yet been exhaustivelyexplored for detection.

Acknowledgements This work was performed under the auspices of the US Department of Energy NationalNuclear Security Administration. The author thanks Jim Koster, Scott Kinkead, and David Robbins for support.

Glossary

ExplosivesAN ammonium nitrate; NH4NO3

ANFO composition of ammonium nitrate and fuel oilAP ammonium perchlorate; NH4ClO4

C-4 composition of 91 % RDX plus waxes and oilsDADP diacetone diperoxideDATB diamino trinitro benzeneDetasheet composition of PETN and NC with plasticizersDMNB 2,3-dimethyl-2,3-dinitrobutane (an explosives taggant)2,4-DNT 2,4-dinitrotoluneEGDN ethylene glycol dinitrate; nitroglycolHMTD hexamethylenetriperoxidediamineHMX octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; octagenHNS hexanitrostilbeneH2O2 hydrogen peroxideNC nitrocellulose; gun cottonNG nitroglycerine; nitro; glyceryl trinitrate; RNG; trinitroglycerineNM nitromethaneNQ nitroguanidinePBX plastic bonded explosivePBX-9501 plastic bonded explosive with HMXPBX-9502 plastic bonded explosive with TATBPE-4 British Comp C: RDX with waxes and/or heavy oilsPETN pentaerythritol tetranitrate; 2,2-bis[(nitroxy)methyl]-1,3-pro-

panediol, dinitrate

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Picric acid 2,4,6-trinitrophenolRDX hexahydro-1,3,5-trinitro-1,3,5-triazine; cyclonite; hexogenSemtex-H composition of RDX and PETN with heavy oils and rubbersTATB triamino trinitro benzeneTATP triacetone triperoxideTetryl methyl-2,4,6-trinitrophenylnitramineTNM tetranitromethaneTNT trinitrotoluene

OtherAOTF acousto-optic tunable filterCARS coherent anti-Stokes Raman scatteringCCD charge coupled deviceCE capillary electrophoresisCMOS complementary metal oxide semiconductorCNT carbon nanotubeCRDS cavity ring down spectroscopyDAPCI desorption atmospheric pressure chemical ionizationDESI desorption electrospray ionizationDIAL differential absorption LIDARECD electron capture detectorELISA enzyme linked immunosorbent assayFAP fluorescence amplifying polymersFET field effect transistorFTIR Fourier transform infraredGC gas chromatographyGCE glassy carbon electrodeHPLC high performance liquid chromatographyICCD intensified CCDIED improvised explosive deviceIMS ion mobility spectrometryLC liquid chromatographyLIBS laser induced breakdown spectroscopyLIDAR light detection and rangingLOD limit of detectionLU lowest unoccupied (molecular orbital)MCC multicapillary columnMEMS microelectromechanical systemsMIMS membrane introduction MSMS mass spectrometryNATO North Atlantic Treaty OrganizationNd:YAG neodymium doped yttrium aluminum garnetPAED photo-assisted electrochemical detectionPDMS poly(dimethyl siloxane)PL photoluminescencePTFE poly(tetrafluoroethylene)QCM quartz crystal microbalanceREMPI resonance enhanced multiphoton ionizationROC receiver operating characteristic

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SAM self assembled monolayerSAW surface acoustic waveSBSE stir-bar sorption extractionSERS surface enhanced Raman scatteringSERRS surface enhanced resonance Raman scatteringSESI secondary electrospray ionizationSFE supercritical fluid extractionSIMS secondary ion MSSLE solid-liquid extractionSPE solid phase extractionSPIE the International Society for Optical EngineeringSPME solid phase microextractionSPR surface plasmon resonanceSTP standard temperature and pressureTDS time domain spectroscopyTEEM tunable energy electron monochromatorTOF-MS time of flight mass spectrometryUV ultravioletUXO unexploded ordinance

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Relevant Conference ReferencesA. J. Sedlacek III, R. Colton, & T. Vo-Dinh (Eds.) Chemical and biological point sensorsfor homeland defense. In Proceedings of SPIE Vol 5269. SPIE, Bellingham, WA.

Edward M. Carapezza (Ed.), Sensors, and command, control, communications, and intelli-gence (C3I) technologies for homeland security and homeland defense III. In Proceedingsof the SPIE Vol. 5403. SPIE, Belllingham, WA.

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R. Jennifer Hwu, & Dwight L. Woolard (Ed.), Terahertz for military and security applica-tions II. In Proceedings of the SPIE Vol. 5411. SPIE, Belllingham, WA.

Russell S. Harmon, J. Thomas Broach, & John H. Holloway, Jr., (Ed.), Detection and reme-diation 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 intelli-gence (C3I) technologies for homeland security and homeland Defense IV. In Proceedingsof 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 Proceedingsof the SPIE Vol. 5781. SPIE, Belllingham, WA.

R. Jennifer Hwu, Dwight L. Woolard, & Mark J. Rosker (Ed.), Terahertz for military andsecurity applications III. In Proceedings of the SPIE Vol. 5790. SPIE, Belllingham, WA.

Russell S. Harmon, J. Thomas Broach, & John H. Holloway, Jr. (Eds.), Detection andremediation 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 basedbiological and chemical sensing, and optically based materials for defense. In Proceedingsof SPIE Vol. 5990. SPIE, Bellingham, WA.

James O. Jensen, & Jean-Marc Thériault (Eds.), Chemical and biological standoff detectionIII. In Proceedings of the SPIE Vol. 5995. SPIE, Belllingham, WA.

Edward M. Carapezza (Ed.), Sensors, and command, control, communications, and intel-ligence (C3I) technologies for homeland security and homeland defense V. In Proceedingsof the SPIE Vol. 6201. SPIE, Belllingham, WA.

Theodore T. Saito & Daniel Lehrfeld (Eds.), Optics and photonics in global homelandsecurity II. In Proceedings of the SPIE Vol. 6203. SPIE, Belllingham, WA.

Michael J. DeWeert, Theodore T. Saito, & Harry L. Guthmuller (Eds.), Photonics forport 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.), Terahertzfor military and security applications IV. In Proceedings of the SPIE Vol. 6212. SPIE, Belll-ingham, WA.

J. Thomas Broach, Russell S. Harmon, John H. Holloway Jr. (Eds.), Detection and remedi-ation 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 crimefighting II. In Proceedings of the SPIE Vol. 6402. SPIE, Belllingham, WA.

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