Report
Detecting Hidden
Explosives D
etecting hidden explosives is essential for areas or installations that are likely targets for bomb attacks, such as baggage control areas at airports, government buildings, industrial plants, and postal service facilities. Bombs vary widely in size, shape, and material, and experience has shown that they can easily be concealed in the harmless objects of everyday life that we don't even notice. Bombs can be detected by recognition of their typical working parts (e.g., timers, switches, detonators) or by recognition of explosive material. These two approaches require different technologies and rely to some extent on the expectation of a standard bomb design. Impro-
Peter Kolla Bundeskriminalamt (Germany) 184 A
Detecting bombs hidden in everyday objects requires a combination of radiation-based and vapor-based methods vised explosive devices with odd construction schemes are particularly hard to recognize. Because metals are common in everyday objects, metal detection alone is not considered conclusive evidence of an explosive device. X-ray imaging can be used to identify the typical constituents of
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bombs in common, ordinary-looking containers. However, if there are several metallic objects that shield one another in the container, or if the container contains metallic shielding or electronic devices, a positive identification cannot be made. Positive recognition by X-ray imaging alone of bombs hidden in radios or computers is impossible. The radio or computer disguising such bombs may even operate without any loss of function. The energetic material itself must therefore be detected. Explosives currently can be detected using either radiation-based or vaporbased detection. The aim of both methods is to respond specifically to the properties of the energetic material that distinguish it from harmless materials of similar composition. A summary of the techniques that will be discussed in this Report is given in Table 1. 0003-2700/95/0367-184A/$09.00/0 © 1995 American Chemical Society
Explosives in general
To achieve the intended effect when tar geting vehicles or people, the amount of explosive material required for a bomb is more than half a pound. Explosives de liver their high energy by a very fast inter nal redox reaction initiated by a lowenergy step. Reduction and oxidation can take place in the same molecule, as in trini trotoluene (TNT), or through the mix ture of an oxidizer and a fuel such as am monium nitrate fuel oil. If the oxidizing and reducing groups are in the same molecule, the reaction proceeds ex tremely quickly and the compound is considered a high explosive. Tradition ally, nitro compounds have been used for these types of reactions. Compounds containing nitro groups, such as nitrate esters (e.g., nitropenta, ni troglycerin, ethyleneglycol dinitrate), nitramines (e.g., trimethylene trinitramine), and nitroaromatic compounds (e.g., TNT) are commonly used in explo sives because they decompose very quickly (after ignition), delivering a large amount of gaseous products. For exam ple, nitropenta and hexogen are used in plastic explosives, TNT is used widely in weapons of war, and nitroglycerin and eth yleneglycol dinitrate are essential compo nents of dynamite. Other types of explosives include mix tures of inorganic salts (with a high oxidiz ing potential) and flammable organic ma terials or metal powder; heavy metal salts such as lead azide, lead styphnate, and mercury fulminate; and nitrate salts that can be mixed with carbon carriers or metal powder (for fuel). Although the energetic materials used for commercial or military purposes have well-known properties, a wide variety of compounds are used in homemade ex plosives. Most such improvised explo sives are mixtures of strong oxidizers, including chlorates, perchlorates, and per manganates, with fuels ranging from mineral oil, vegetable oil, and sugar to
metal powders, sulfur, and phosphorus. Most of these mixtures are very impact- or friction-sensitive and do not explode fast enough to be used as bombs without be ing confined. Radiation methods
Because explosives can be hidden inside various types of containers filled with many different materials, effective radia tion methods are limited to penetrating electromagnetic radiation, low-energy ra diation radio waves and microwaves, and high-energy radiation X-rays and γ-rays. Although radiation methods that depend
Bombs can easily be concealed in the harmless objects of everyday life that we don't notice. on compound-specific interactions would allow the differentiation of organic explo sives from most ordinary everyday sub stances, these methods cannot be used because they do not penetrate the con tainer. NMR, NQR, and ESR. The applica tion of NMR spectroscopy to the detection of hidden explosives requires an exten sive change in technical specifications be cause the classical measurement of the chemical shift of XH nuclei is defeated by the mixture of signals from other materi als in the container. Relaxation of the 1H nuclei is more specific for explosives in a complex environment, but the sensitivity required for detection of the small amount of explosives in suitcases and the limited measurement time available for each
check can lead to false detection of explo sives (i). Because of the high content of 14N nu clei in explosives, the use of nuclear quadrupole resonance (NQR) spectroscopy, an NMR-like method based on 14N rather than 'H, has been investigated for detec tion of explosives (2). Although NQR is more specific than XH NMR for detect ing explosives, reliable detection of differ ent types of explosive materials in com plex matrices is still a problem. The application of electron spin reso nance (ESR) spectroscopy to the detec tion of hidden explosives is limited be cause most explosives have no unpaired electrons. However, ESR spectroscopy has been found to be very sensitive for de tecting black powder (3). All of these methods suffer from inter ferences, and the false detection rate in creases tremendously if the equipment is used to recognize several different types of explosives simultaneously. In addition, the strong magnetic field needed for NMR or ESR techniques may destroy mag netic tapes or disks present in the article being examined. The most disturbing problem with these methods, however, is the ease with which explosives can be shielded by wrap ping them in metal foil. Despite these dis advantages, these methods are useful in certain instances because they can be au tomated, provide chemical identification of pure bulk explosives (using NMR and NQR techniques), and can be used to de tect liquid explosives. X- and y-rays. Because X-rays and γ-rays penetrate most materials, emission, absorption, or X-ray diffraction analysis is used for the detection of hidden explo sives (4). In emission analysis, direct bom bardment with electrons cannot be used because the electrons are absorbed at the surface of the container. Short-wave length X-rays can penetrate these surfaces and the material surrounding a hidden explosive, but the intensities of the result-
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Report ments with high atomic numbers, such as metal salt primary explosives or impro vised explosives with large amounts of Technique Basis for detection Type of explosive(s) chlorine or metal powders. Because there is no strict elemental resolution and no Radiation techniques total qualitative or quantitative analysis, NQR, NMR, and ESR Structural fingerprint Organic X-ray absorption Heavy metal content, Inorganic, improvised, the positive identification of these explo density organic sives is not reliable; harmless material can X-ray emission Inorganic, improvised Heavy metal content give a false positive. X-ray absorption X-ray diffraction Solid forms of organic, Crystalline structure inorganic, and does not selectively detect organic explo inorganic, and improvised improvised sives containing only light elements, nor γ-ray absorption Nitrogen density Organic nitro, inorganic, Organic nitro, inorganic, does it clearly differentiate between ex improvised improvised Thermal neutron activation Nitrogen density Organic nitro, nitrate salts, plosives and other organic materials such Organic nitro,improvised nitrate salts, inorganic, as plastic or foods. inorganic, Fast neutron activation Carbon, oxygen, nitrogen Organic nitro,improvised inorganic, The advantage of modern X-ray absorp densities Organic nitro, inorganic, improvised improvised Vapor techniques tion techniques is the excellent imaging Molecular structure Organic MS that can be obtained of the inner contents Ion mobility spectrometry Mobility of ionized Organic of an object. A potential explosive can be mn|pf*ii|pQ molecules recognized by its shape and then further GC with electron capture Chromatographic properties Organic Chromatographic properties and detection investigated by a more selective method. and electron electron affinity affinity GC Chromatographic GC with with chemilumineschemiluminesChromatographic properties properties Organic nitro These techniques are also easy to use and and presence of nitro cence detection detection cence and presence of nitro groups can possibly be combined with other de groups tection systems. X-ray diffraction. Recognition of explosives by X-ray diffraction should be ing fluorescent X-rays are more than 1000 are therefore not very useful for detecting very selective because most explosives are crystalline, and the combination of spe times smaller than those from an X-ray organic explosives. beam obtained by direct excitation with a For absorption analysis without energy cific chemical and X-ray characterization beam of electrons, making very sensitive resolution, the transmittance depends not appears promising. The reliability of the method strongly depends on the geo detectors necessary. For organic nitro only on the mass absorption coefficient metrical resolution. For explosives detec explosives, this low-intensity fluorescence but also on the thickness of the material tion, a fixed scattering angle and polychro prevents X-ray emission spectroscopy being examined. Thick, low-absorbing ma matic X-radiation are used. The biggest from being used for elements lighter than terials result in absorption similar to thin, problem of the technique is differentiating magnesium. high-absorbing materials. To overcome the signals from nonexplosive materials, X-ray absorption spectroscopy is used this problem the dual energy method (5, potential explosives, and background to check the inside of a container that may 6) either uses two types of detectors or noise from absorption effects. Spatial be dangerous or impossible to open, al shields part of the detectors with highscanning of the suspect container is a pos though this classical technique gives no absorbing material. Light materials ab detailed information about the material. sorb only low-energy X-rays; heavier mate sible solution. Gamma radiation. A nucleus that However, factors that contribute to the rials absorb both low- and high-energy Xmass absorption coefficient (the photo rays. The ratio between the transmittance absorbs a γ-photon of a given energy can fluoresce at the same energy (8). The mea electric effect, classical scattering, and the value at high and low energies can be Compton effect) can be used to enhance used to determine the type of material; this surement of the absorption strength gives information on element concentra explosive-specific detection. ratio is high for organic materials and tion in the radiated object. For example, it In X-ray machines used at security lower for materials made of heavier ele is possible to determine the concentra checkpoints, transmittance is measured. ments. tions of nitrogen, carbon, and oxygen, Maximum information is obtained using To take advantage of the Compton ef which have different resonant absorption energy-dispersive analysis. Qualitative fect, in which a photon interacts with an elemental analysis can be obtained from electron, loses energy, and changes its ini energies. However, γ-ray absorption suf fers from interferences from harmless or energy-dispersive spectra because the tial direction, backscattered X-rays are of ganic materials and low sensitivity. photoelectric effect results in absorption ten measured (7). Compton backscatteredges that are typical for different ele ing gives information about light materi The most promising methods explored ments. However, because energy-dis als in the object when they are not covered in the past few years for the detection of persive machines are not optimized for ex by heavy materials. explosives are based on neutron activation plosives detection and are susceptible to analysis (9-12). Elemental analysis is The usefulness of X-ray absorption numerous interferences, they detect only performed by measuring the generated γ methods is primarily limited to the detec elements with high atomic numbers and tion of hidden explosives that contain ele emission response of the sample material T a b l e 1 . S u m m a r y of t e c h n i q u e s
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after activation by fast or thermal neu trons. The energy of the activating neutrons and the nuclear reaction involved deter mine which of two types of neutron activa tion analysis are used. Thermal neutron activation (TNA) is based on the selective emission of 10.8-MeV γ photons, one of the highest possible energies for such a re action (10). Because the intensity and spatial distribution of these high-energy γ photons is a strong indication of the pres ence of nitrogen (13), extraordinarily high nitrogen content can be used to detect hidden organic explosives. Unfortunately, however, explosives are not the only compounds with high nitrogen content; polymers with an amide or urethane struc ture, as well as proteins, can interfere with the method. Fast neutron activation (FNA) has the advantage of being able to detect nitro gen, carbon, and oxygen. The production of γ photons is based on the inelastic scat tering of neutrons at the nuclei. Pulsing the neutron flux lowers the background ra diation. Combined with time coordina tion of the neutron input, γ photon detec tion and the position of the suspect con tainer on a conveyor belt make spatial resolution possible. Because it can pro vide the ratios of nitrogen to oxygen and nitrogen to carbon, pulsed FNA makes the number of false positives much lower (14, 15). The greatest disadvantage of these high-energy methods is the need to generate the necessary MeV radiation or neutrons. Vapor methods
In contrast to radiation-based methods, va por detection is noninvasive and mea sures traces of characteristic volatile com pounds that evaporate from the explo sive or are present on the container surface. However, vapor detection can take much longer than radiation detec tion. Vapor detection is restricted to com pounds with sufficiently high vapor pres sures. Detection should be possible if some of the explosive was deposited on the surface of the container during pack ing. This surface contamination enor mously facilitates detection and is often the reason for successful detection of va por from explosives. However, the method
is often capable of detecting vapor from in side the package when none is present on the surface. Because inorganic salts have negligi bly low vapor pressure, only the organic constituents of explosives can be detected using vapor detection (16). The explo sive oils nitroglycerin and ethyleneglycol dinitrate have such high vapor pressures (at the part-per-million and upper partper-billion levels) that explosives contain ing one of these oils have a distinctive odor, and no sophisticated equipment is necessary for its detection. The detectability of the other compo nents is dependent on the sensitivity of the equipment to vapor concentration. The vapor pressure of dinitrotoluene and TNT is at the part-per-billion level, but the va por pressure of hexogen and nitropenta is
tives. The most accurate identification can be made when structural information on the molecule is available. MS. MS does not appear to be suffi ciently selective for trace analysis of explo sives because simple nitrate esters such as ethyleneglycol dinitrate, nitroglycerin, or nitropenta do not exhibit sufficient frag mentation to distinguish themfromnonexplosive substances contained in the bomb. The moderate sensitivity of MS combined with its high cost have pre vented it from becoming a widely used technique for detection of explosives. Ion mobility spectrometry. IMS is of special interest for the detection of ex plosives because of its very high sensitiv ity to traces of electron-capturing organic substances such as compounds with nitro groups. The selectivity of IMS is based on the combination of ionization of electro negative species and detection at evalu ated drift conditions. IMS systems with simple ionization in the environmental gas and detection of one sample peak often give false positives because of interfer ences from everyday substances with simi lar IMS behavior. More sophisticated IMS systems per form multiple peak detection after forma tion of multiple ions using special drift gases. The applicability of IMS to the de tection of organic explosives is good, par ticularly because compounds other than nitro explosives (such as peroxides) can in the low part-per-trillion range. If one of be detected. GC. Because ordinary GC separations these substances is a component of the ex plosive at < 50% mixed with inorganic ex are too slow for practical explosives detec plosives, the vapor pressure is decreased, tion in the field, specialized chromatomaking detection more difficult. If the ex graphs that use very short columns and high gas-flow rates are used. Temperature plosive is wrapped in plastic or cloth, de programs are also ramped quickly or the tection is even more difficult because the vapor must diffuse through the wrapping. systems are operated isothermally to opti mize detection. Electron capture detec In light packaging, sensitivity must be in tors, which are very sensitive to com creased by a factor of ~ 1000 to compen sate for this decrease in vapor availability. pounds such as chlorides, esters, and ni tro compounds, are usually used. Equipment capable of detecting compo nents with vapor pressures similar to that Chemiluminescence detection based of TNT must have a sensitivity at the low on the formation of nitrogen oxide by highpart-per-trillion level. temperature pyrolysis of nitro com Selectivity is one of the most important pounds has been successfully used for trace analysis of explosives in the forensic factors in vapor analysis. Many sub laboratory (17,18). The nitrogen oxide is stances in the environment of a checked reacted with ozone in a special detection object have a significantly higher concen tration than the part-per-trillion range and chamber to form excited nitrogen diox may interfere either by masking the pres ide, which emits 600-nm photons when re turning to the ground state. The light is ence of explosives or by giving false posi
The most promising methods explored recently for the detection of explosives are based on neutron activation analysis.
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Report
detected and amplified by a photomultiplier. This method has been effectively integrated in an explosive detection system and can recognize those compounds that pyrolyze to nitrogen oxide, such as nitro and nitroso compounds. Advantages and disadvantages. With the exception of MS, these methods are all used in commercially available vapor detectors for explosives. IMS is better suited than MS because it can be very selectively tuned to detect explosives. There still may be false positives, however, if the instrument is overloaded with perfume or scented toiletries. GC with electron capture detection is the least reliable of these methods because the selectivity in complex samples with large amounts of accompanying substances is low and the sensitivity for compounds other than explosives is high. GC with chemiluminescence detection is well suited for organic nitro constituents, but false positives resulting from fumes containing nitroaromatic compounds are also possible. Substances with nitrogen and oxygen in the same molecule, although not necessarily in the form of nitro or nitroso groups, also can cause false positives. The biggest disadvantages of all the vapor detection methods are that they are limited to organic explosives and that the explosive be vaporizable. Sampling. The most important (and time-consuming) step in explosives detection is vapor sampling (19-21). The detector, no matter how sensitive, can detect hidden explosives only if the sampling procedure collects enough vapor. Evaluation of any vapor detection equipment must always include thorough testing of both the analytical method and the sampling device. A gas adsorption device samples only the more volatile components, but not hexogen and nitropenta, which are of special concern because they are the main constituents of easily concealed plastic explosives. Because explosives adhere strongly to many surfaces, however, they can be collected by wiping with a solventwetted cotton swab (22) or a tissue, which is then analyzed. Tissue analysis gives the best detection limit. The only problem is getting the sample off the wiping material and into the gas phase for analysis. This can be 188 A
done by heating the tissue or swab and collecting the vapor, fibers, and microparticles in a special trap that can be heated to a higher temperature to get the components into the analytical system. If the wiping material is heated and the components are injected directly into the analytical system, the tissue or swab must be small and made of a special material suited to higher temperatures; an inert filter system is also needed to prevent large fibers and big particles from clogging up the works. Method selection
Radiation methods are best suited for examining objects in places where portabil-
ity of the instrument is not an issue, such as security checkpoints at airports. Explosive vapor detectors, which are at least movable if not totally portable, are not suited for automatic routine screening; they need too much time and several manual handling steps for proper operation and are best suited for examining selected objects. The two techniques complement one another—a vapor detector cannot completely replace a radiation system and vice versa. Although most explosives have a characteristically high nitrogen content, the threat of low-nitrogen-content explosives is not negligible. Improvised explosives made from nitrate salts mixed with fuels contain large amounts of nitrogen but have no volatile nitro constituents. If the oxidizer material is a chlorate or perchlorate, there is no nitrogen at all. Nitrogen also is absent from some improvised organic
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explosives such as peroxides. Though mixtures of chlorate or perchlorate with moderate fuels such as oil or sugar must be confined for detonation, mixtures such as perchlorate and aluminum powder or chlorate and phosphor can be detonated without confinement. The success of vapor detection is highly dependent on the presence of trace explosives on the outside of the bomb container. Vapor detectors can detect only organic nitro compounds that can be evaporated without decomposition; all other explosive materials such as salts, primers, or peroxides cannot be detected. The sensitivity is not related to the total amount of explosive but to the amount of contamination on the sampled surface. Spatial resolution to locate a positively detected explosive is not possible. All systems that detect nitrogen should be able to positively detect most explosives. The reliability of detection depends on the preset detection threshold, the total amount of explosive, and the geometry of the explosive. Problems arise when the explosive is distributed throughout the object. For example, detection may fail in the case of plastic explosives in a flat or sheet form. Explosives in such shapes are detectable only if the technique—such as pulsed FNA—is capable of spatial resolution. Other methods such as X-ray absorption produce 3D images only with a second source and second detector array. Obviously, nitrogen detectors are not suited to finding non-nitrogen-containing improvised explosives, but they can detect most of the primers. A broader spectrum of explosives, including those that are improvised, can be detected if chlorine is included in the analysis scheme. The mere differentiation among groups of elements is likely to give false positives and thus is not suited for the detection of explosives. Detection based on high nitrogen content alone often gives false positives because of possible interferences from nitrogen-based plastics or other organic materials with a higher nitrogen content than the explosives themselves. Though the density of the nitrogen in the irradiated region is used to increase selectivity, the informational content of nitrogen detection alone is low. Better selec-
tivity, and a corresponding decrease in false positives, can be achieved by combining nitrogen detection with oxygen and carbon detection, such as in pulsed FNA, along with simultaneous spatial resolution. Methods that detect substances rather than specific elements should be able to detect all explosives on the basis of their individual characteristics. One problem, however, is that sensitivity for molecular recognition is lower than that for elemental recognition. This lower sensitivity can be compensated for, however, by spatial resolution. The concentration of the explosive should then be high enough to get a clear identification.
The techniques complement one another—vapor detection can't completely replace radiation detection and vice versa. The biggest advantage of substance identification methods is that they are independent of the nitrogen content or of nitro groups. Even explosives such as peroxides or mixtures of strong oxidizers other than nitrates can be detected if their structure is known, making these systems among the most universal for the detection of explosives. For explosive vapor detectors, if the nitro group is used only for the selection of electron-capturing molecules, interferences from chlorides and esters are possible. If these interfering compounds are present at very high concentrations, such as in perfumes, overloading effects may disturb the physicochemical selection by chromatography or IMS and result in a false positive. Selectivity is gained by using methods that detect specific behaviors of nitro groups (such as chemiluminescence) if these are cou-
sium on Explosive Detection Technology; Federal Aviation Administration: Atlantic City, NJ, 1991; p. 269. (8) Metzger, F. Progress in Nuclear Physics 1989, 7, 54. (9) Gozani, T.; Morgado, R. E.; Seher, C. /. Energ. Mater. 1986,4,377. (10) Gozani, T.; Morgado, R; Seher, C. Proceedings of the 3rd International Symposium on Analysis and Detection of Explosives; ICT: Karlsruhe, Germany, 1989; p. 36-1. (11) Gozani, T.; Shea, P. M. In Advances in Analysis and Detection of Explosives; Yinon, J., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; p. 335. (12) Gozani, T. Proceedings of the 1st International Symposium on Explosive Detection Technology; Federal Aviation Administration: Atlantic City, NJ, 1991; p. 27. (13) Buchsbaum, S. B.; Knize, D.; Feinstein, L; Bendahan, J.; Shea, P. Proceedings of the 1st International Symposium on Explosive Detection Technology; Federal Aviation Administration: Atlantic City, NJ, 1991; p. 70. (14) Sawa, Z. P.; Gozani, T. Proceedings of the 1st International Symposium on Explosive Detection Technology; Federal Aviation Administration: Atlantic City, NJ, 1991; p. 82. (15) Vourpoulos, G.; Schultz, F J.; Kehayias, J. Proceedings of the 1st International Symposium on Explosive Detection Technology; Federal Aviation Administration: Atlantic City, NJ, 1991; p. 104. (16) Tamiri, T.; Zitrin, S.; Abramovich-Bar, S.; Bamberger, Y; Sterling, J. In Advances in Analysis and Detection ofExplosives; YÏnon, J., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; p. 335. (17) Fine, D. H. Proceedings of the 1st InternaReferences tional Symposium on Analysis and Detec(1) King, J. D.; DeLosSantos, A; Nicholls, C. tion of Explosives; FBI Academy: QuanI.; Rollwitz, W. L. Proceedings of the 1st Intico, VA 1983; p. 159. ternational Symposium on Explosive De- (18) Kolla, P.J. Forensic Sci. 1991,36,1342. tection Technology; Federal Aviation Ad- (19) Jenkins, A; McGann, W.; Ribeiro, Κ Pro ministration: Atlantic City, NJ, 1991; ceedings of the 1st International Sympo p. 478. sium on Explosive Detection Technology; (2) Buess, M. L.; Garroway, A. N.; Miller, Federal Aviation Administration: Atlantic J. B.; Yesinowski, J. P. In Advances in City, NJ, 1991; p. 532. Analysis and Detection of Explosives; Yi- (20) Bromberg, E. A; Carroll, A. L.; Fraim, non, J., Ed.; Huwer Academic Publishers: F. W.; Lieb, D. P. Proceedings of the 1st In Dordrecht, The Netherlands, 1993; p. 361. ternational Symposium on Explosive De (3) Poindexter, E. H.; Lenpold, H. A; Wittstection Technology, Federal Aviation Ad truck, R. H. Proceedings of the 1st Internaministration: Atlantic City, NJ, 1991; tional Symposium on Explosive Detection p. 552. Technology; Federal Aviation Administra- (21) Fraim, F. W.; Achter, Ε. Κ; Carroll, A L.; tion: Atlantic City, NJ, 1991; p. 493. Hainsworth, E.; Little, A. D. Proceedings of (4) Grodzins, L. Proceedings of the 1st Internathe 1st International Symposium on Ex tional Symposium on Explosive Detection plosive Detection Technology; Federal Avia Technology; Federal Aviation Administration Administration: Atlantic City, NJ, tion: Atlantic City, NJ, 1991; p. 201. 1991; p. 559. (5) Dolan, K. W.; Ryon, R. W.; Schneberk, (22) Kolla, P.; Sprunkel, A/. Forensic Sci., in D. J.; Martz, H. E.; Rikard, R D. Proceedpress. ings of the 1st International Symposium on Explosive Detection Technology, Federal Peter Kolla is a forensic scientist at the Aviation Administration: Atlantic City, NJ, Bundeskriminalamt (national law enforce 1991; p. 252. (6) Krug, K. D.; Stein, J. A Proceedings of the ment agency for Germany) and can be 1st International Symposium on Explosivereached at KT16, 65173 Wiesbaden, Ger Detection Technology; Federal Aviation Ad-many. His research focuses on the analysis ministration: Atlantic City, NJ, 1991; and detection of explosives and explosive p. 282. devices, including those that are home (7) Schafer, D.; Annis, M.; Hacker, M. Proceedings of the 1st International Sympo- made.
pled to an effective chromatographic separation. Nonexplosive substances containing nitro or nitroso groups, or even a large amount of nitrogen and oxygen in the same molecule, may exhibit chromatographic retention similar to explosives. The most serious disadvantage of explosive vapor detectors, however, is the lack of spatial resolution. Only the surface of a container is sampled; in the case of an alarm the explosive cannot be easily located to determine whether the alarm is real. The greatest limitations in detecting hidden explosives arise from the need to quickly check closed containers, as at the baggage security checkpoint at airports. Suitcases moving through on a conveyor are best checked by radiation techniques, whereas explosive vapor detectors are restricted to detailed checks of baggage containing suspected explosives. Efficient equipment exists with the capability to detect small amounts of explosives in different shapes, but not for all possible explosives simultaneously. The best approach is the combination of several methods that complement each other.
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