Microencapsulated
detection taggants shown with a needle to indicate size
EXPLOSIVES During 1978, 23 people were killed and 185 wounded by explosive and incendiary bombs, according to statistics of the Bureau of Alcohol, Tobacco and Firearms. The bureau estimates that property damage in that same year was at least $17 million. Approximately 4 billion lb of explosives are manufactured and used annually in the U.S. At the request of a Senate committee, Congress's Office of Technology Assessment (OTA) recently undertook a study of a proposal to require taggants in explosives. Taggants are substances that would be added to explosives at the time of manufacture as an aid to law enforcement. The published OTA study, "Taggants in Explosives," assesses taggant technology in order to assist Congress in deciding whether or not to mandate their use in commercial explosives manufactured in the U.S. Legislation proposed in the Congress would make it unlawful for any person or persons to manufacture an explosive material that does not contain both identification taggants and detection taggants. Identification taggants are designed to be retrieved from the debris of an explosion. They would contain a
code identifying the batch of explosives used in the bombing. Thus, law enforcement investigators would know what kind of explosive material had been used, and would be able to obtain a list of legal purchasers of the materials. Detection taggants, on the other hand, make it possible to detect the presence of concealed explosives. The taggant material is designed to be sensed by a detector, even when the explosives are contained in a package inside some luggage. Detectors located at sites such as airports and nuclear power plants could then signal any effort to introduce explosives into the area. In facilities lacking permanent detectors, portable sensors could be used to sniff out explosives where there was a bomb scare. The detection taggant itself would probably be a microcapsule which would emit small quantities of a vapor. Detection taggant technology, as presently conceived, involves the use of some sophisticated analytical methodology and instrumentation. The report emphasizes the potential effectiveness of detection taggants in protecting very high value targets. But it also points out that the system would by no means be foolproof.
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Countermeasures by terrorists could be effective if they were carried out with sufficient knowledge and skill. Identification Taggants Currently favored as identification taggants are microscopic plastic chips developed by the 3M Company. Approximately 0.12 mm thick and about 0.40 mm long, the chips, made of thermosetting melamine alkyd, have eight layers, each a different color. This provides a total of approximately six million possible color codes. To aid in recovery, one face of the taggant fluoresces in the visible region when illuminated with black light (366 nm), and the other face contains Fe powder, allowing the taggant to be picked up by a magnet. In theory, the taggant can be recovered from bomb debris with a black light and a magnet, read with a low-power microscope, and decoded through a tracing center at the Bureau of Alcohol, Tobacco and Firearms. Detection Taggants Prior to the development of detection taggants, a number of techniques for sensing untagged explosives were investigated, but the lack of a common vapor in the various explosive materi-
Focus
als hampered these efforts. A detection device would have had to detect a variety of vapors to be generally applicable, and would thus have been quite slow, quite expensive, or both. The detection taggants currently under investigation were chosen from among hundreds of candidate chemicals. These substances were screened in a search for a vapor that is scarce in nature, and which exhibits long-term stability, chemical inertness, appreciable vapor pressure, and nonadhesion to surfaces likely to be present in containers used to conceal bombs. Currently under serious consideration is a series of perfluorinated alkanes. Five of these compounds have successfully passed barrier penetration, mutagenicity, toxicity, and atmospheric impact testing. They are: perfluorodimethyl cyclobutane, perfluoromethyl cyclohexane, perfluorodimethyl cyclohexane, perfluorodecalin, and perfluorhexyl-sulfur-pentafluoride. As yet, no specific taggant among the five candidate molecules has been chosen, and an encapsulation method would have to be worked out. The OTA report indicates that the technology for encapsulated detection taggants probably could not be at the production stage much earlier than 1985. Detection Sensors A detection taggant is only good if there is a sensor available to determine its presence. Three sensor types are being considered for use with the microencapsulated detection taggants. The ion mobility spectrometer (IMS) has been commercially available for about five years, with approximately 50 instruments currently being used for laboratory analysis. The continuous electron capture detector (CECD) has also been produced as a laboratory instrument in limited quantities. And the mass spectrometer (MS) under evaluation is a simplified version of the standard laboratory instrument. In the field, a device would apply a pressure pulse to a piece of baggage to force some of the air from the interior into the detector inlet. If a vapor taggant were detected by the sensor, an alarm would be activated.
In the IMS spectrometer, the sample is first stripped of free oxygen and water vapor in a conditioner. Any molecular species present, including taggant molecules, are ionized in an ion-molecule reaction chamber, and then injected into a drift tube, where an electric field causes them to flow against a countercurrent stream of drift gas. The ions are separated into clumps of like species in the process of reacting with the neutral drift gas molecules. Each species traverses the drift tube in a different length of time. The taggant molecules have long drift times, and are easily separated from common gases at this stage. Additional specificity is realized because the taggant molecules remain intact in the detector—most other large molecules fragment in the process. Detection is accomplished with a fast electrometer amplifier. The CECD can be viewed as an IMS device without the drift tube. It simply consists of a conditioner and a reaction chamber. A decrease in current in the reaction chamber as the perfluorinated taggant molecules "capture" electrons indicates their presence. In the CECD unit under study, oxygen, water vapor, and other potential interferents are fractured in a specially designed conditioner. Still other interferents are removed by combustion or reduction. However, the taggants still may not be the only survivors of the passive screening process, and the CECD is less discriminating than the IMS. The MS is easily capable of resolving the taggant molecules from other species. However, MS instruments are expensive and relatively temperamental. The challenge is to design a lowcost version, suitable for use by unskilled persons in the field, and capable of detecting taggants at ppb levels. So far, none of the sensors has been proven feasible or chosen for exclusive development. OTA estimates it would take at least five years to get an instrument into the production stage. Prospects for Legislation At hearings on the proposal, representatives of the explosives and gunpowder industries, and other critics of
the taggants idea, have expressed their opposition to the concept. They object to the addition of a foreign substance to explosive materials, which they feel may make them more unsafe. They also claim that the taggant program would be extremely costly, and that it would not have much utility for law enforcement. Proponents have countered that taggants are inert materials, no more unsafe than additives currently used in explosives; that a taggants program need not be unduly costly; and that bombings are extremely difficult crimes to prevent and solve using existing methods, a difficulty that use of taggants might ameliorate. The OTA report concludes that identification taggants would facilitate the investigation of almost all significant criminal bombings in which commercial explosives were used, and that detection taggants would be very effective in protecting high-value targets where use of detection taggant sensors would be economically feasible. Two antiterrorism bills introduced in this session of Congress contained provisions mandating the use of taggants: S. 333 and H.R. 2441. It is expected that both bills will be passed this year, but not necessarily with taggants provisions. When the House bill was reported out of the Committee on Public Works and Transportation the taggants provisions had been excised. And the possibility exists that they will be removed in the Senate as well. One House aide explained, "There have been questions raised about safety, cost, and effectiveness by the National Rifle Association, other gun groups, and some of the chemical manufacturers. The Members have been responsive to these concerns." The only hope for the measure lies in reinstatement by another House committee, reinstatement in a Conference of the House and Senate, or passage in another session of Congress. Copies of "Taggants in Explosives" are available at $6.50 per copy from Superintendant of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Request Stock No. 052-003-00747-9.
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