Recent Advances in Trace Explosives Detection Instrumentation




НазваниеRecent Advances in Trace Explosives Detection Instrumentation
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Recent Advances in Trace Explosives Detection Instrumentation


D. S. Moore

Shock and Detonation Physics Group, Los Alamos National Laboratory, Los Alamos, NM 87545 USA


Abstract


There has been a huge increase in instrument development for trace detection of explosives in the past three 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.


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 stand-off 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.


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 Figure 1, revised and updated from Figure 1 of Reference 1. Some common explosives 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 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].


Recent reviews


A large number of workshops and conferences have been held in the past three 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 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.



Figure 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.


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.



Figure 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 Figure 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.


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


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.


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.


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].


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 C8- ions has been used to analyze samples of explosives dispersed as 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 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].

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