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The government-mandated addition of chemical "taggants" to gunpowder and explosives has often been proposed in the U.S. as a legislative strategy for combatting terrorist bombings, by giving such potential bombmaking materials a unique chemical "signature" which could provide investigators additional clues to work from in identifying bombing suspects. Recent high-profile bombing incidents, such as those at the Alfred P. Murrah Federal Building in Oklahoma City, New York City's World Trade Center, as well as the Atlanta Olympic Park bombing, and bombings at abortion clinics and elsewhere, have given new political currency to the "taggant" issue, which was last seriously debated by the U.S. government during the late 1970s. As the public treatment of former Atlanta Olympic security guard Richard Jewell shows, assigning blame for terrorist bombings is at best an inexact science given current techniques, so the question remains whether "taggants" will help or hinder the cause of justice and public safety.
There are three technologies presently competing for the "taggant" market, all of which are in current use in other products, primarily to help identify counterfeit merchandise, such as bogus aircraft parts, gasolines, cosmetics, medicines, and even alcoholic beverages. In this role of assuring the quality and integrity of brand-name products, these covert identification methods may in fact help save far more lives than they ever could as additives to explosive materials. The earliest "taggant" technology, originally developed at 3M, is the one most often discussed when the issue is tagging explosives and gunpowder, since it was the method under consideration in a 1980 report by the Congressional Office of Technology Assessment. The "taggants" in this case are tiny flecks of melamine plastic, which contain color coded layers sandwiched between a magnetic film on one side, and a fluorescent layer on the other side which glows under ultraviolet (UV) light. Detectives at a bombing scene, the theory goes, would use UV light to find the glowing taggants, collect them with a magnet, and read the color-coded layers under a microscope to trace the serial number of the batch of explosive used to make the bomb.
Another kind of tagging method uses biotechnology derived from the immune systems of animals to detect the presence of specific chemicals attached to a larger inert carrier molecule. The chemical tags, called haptens, used in this method would only be identifiable by a specific immune system molecule, a monoclonal antibody, developed to recognize them. A microscopic plastic bead which had several different types of haptens on its surface could be "read" by treating it with the corresponding antibodies, in a similar manner to the way invading bacteria and viruses are recognized as "foreign" by the immune system. Determining which monoclonal antibodies could bind to the beads, and which could not, would identify the haptens present on the bead, and hence the code. The use of such a biological technique allows the taggant particles to be made much smaller, and used and detected in much lower concentrations than taggants using a color-code. Also, the biotaggants would be difficult to remove from the explosive mixture, and to identify, without the proper antibodies.
Lastly, there is a tagging system available which exploits the fact that all the chemical elements can exist in the form of atoms having different slightly different weights, called isotopes. Hydrogen, for instance, can exist as "light" hydrogen, having only an electron and a proton; and "heavy" hydrogen, which has an electron and a proton, but also contains a neutron. (There is also a heavier radioactive isotope of hydrogen, but such a radioactive tag, though easy to detect, would not be ...technically suitable.) Both of these forms of hydrogen are chemically very similar (almost identical), but can be distiguished by using a sensitive piece of scientific equipment called a GC-mass spec (short for gas chromatograph-mass spectrometer), which is capable of weighing atoms and molecules. "Heavy" hydrogen atoms are much rarer in nature than the "light" kind, so substituting particular "heavy" hydrogen atoms for their "light" counterparts in some of the molecules found in an explosive would not change the chemistry of the explosive, but would create a unique pattern that is detectable using the proper instruments, even when present at very low levels. Best of all, such isotopically labeled explosives would not contain foreign materials which might cause environmental and safety problems, the technologies involved in making the tags are not easily duplicated, and the isotopes themselves cannot be destroyed except by nuclear processes which would also destroy whatever materials the tags are associated with.
Putting foreign "taggant" particles into explosives and gunpowder is tricky and possibly very dangerous, and certainly useless for the intended purpose if it can be easily defeated. The color-coded flecks of magnetized plastic have several strikes against them. If they are easy to find and remove from the bomb residue, they may be easy to remove from the explosives beforehand. Treating gunpowder with a magnet could pull them out of the mix. It has been proposed to include some proportion of non-magnetic taggants, and/or conceal the fluorescent layer with a substance that looks similar to the explosive itself, and which would only reveal the fluorecence when it gets burned away in the explosion, so as to make removing the taggants much more difficult. However, unless the plastic particles are of similar size and density to the powder, they could separate due to settling, or be screened out. Settling out could cause changes in loading characteristics when powder is used to manufacture ammunition. For instance, if the load is adjusted to account for a certain proportion of taggant, the first part of a run may be correct, but later, after settling, there may be rounds loaded with high concentrations of taggant, and later still, rounds which are overcharged with too much powder. The taggant itself might react chemically with the highly reactive chemicals present in the powder or explosive, with possibly destabilizing and dangerous results. A 1980 report by the Congressional Office of Technology Assessment noted possible instability resulting from the presence of high taggant concentrations at high temperatures, though it's doubtful whether this data has any relevance at the far lower concentrations in which the taggants would ordinarily be used. If the taggants tend to clump, they may become locally concentrated, however, causing ballistic inaccuracy. If not degradable, microscopic bits of plastic taggants incorporated into fertilizers, explosives and gunpowder would accumulate in the environment, making it difficult to determine what taggants came from where. Mixing several lots of tagged chemicals together as in a "home brew" type improvised explosive, and/or diluting out or substituting untagged chemicals for the tagged ones could defeat the system, or reduce the concentration of the tags to levels indistinguishable from the levels already present as a result of the use of explosives in mining the components of many commonly used materials, such as concrete, mortar, brick, and glass. The most powerful conventional explosives, the so-called "fuel-air" explosives, would not be suitable for labeling with such particles.
Even if the particles were reduced in size, as in the hapten carrier molecules, it remains to be seen whether sufficient numbers of the haptens on the surface of the carriers would retain the shapes and chemical characteristics after an explosion which enable them to be recognized by the highly sensitive monoclonal antibodies. Though the hapten carrier tags would be tough to remove from the explosive, and almost impossible to read without the proper antibodies, there is still the possibility that the particles could react chemically with the explosive, becoming unreadable, or causing destabilization. There is also the possibility that the haptens could be chemically modified in some fashion as to make them unreadable by the antibody, in effect changing the chemical serial number present on the carrier, either as a result of environmental exposure, or human intervention. Treatment of the taggant particles with enzymes that destroy the unique three-dimensional structure of the haptens without damaging the explosive may be possible, depending upon their chemical properties. Putting a protective coating on the hapten carrier particles might prevent these problems, but it would probably also make the particles more difficult for detectives to locate and read, increasing the possibility of investigative errors. Again, substitution or dilution of the tagged chemical could defeat the system, and there is also a concern for accumulation in the environment. While the hapten carrier tags have some advantages, such as increased security, they share many of the drawbacks of the earlier color-coded plastic taggants, in that they introduce a foreign particulate substance into the reaction mix. As anyone who's cleaned a gun knows, there's always residue left behind from incomplete combustion of the powder, so there is the possibility that enough functional haptens can survive an explosion to be useful to an investigation. But buildup of taggants inside a gun might cause problems as well, and if foreign taggant particles tend to cause ammunition to be less accurate, then using tagged ammunition could put the user or bystanders at risk in defensive situations where accurate shooting is critical.
Of all the current taggant technologies, isotopic labelling seems to be able to overcome every possible objection, since no foreign materials are being introduced into the powder or explosive, and the infinitesimal differences at the atomic scale make no difference to the performance of the chemicals they are included in. The isotopes which are used in the process are naturally occurring, and though they may accumulate locally for a while, they would gradually rejoin the natural chemical cycles of the Earth, in the same manner as their lightweight counterparts, with no adverse effect on organisms in the environment. The added labor costs involved in cleaning up after each batch of labeled explosive or powder so as not to cross-contaminate later batches would be passed on to the consumer of all products in which labeled substances are used, no matter what the taggant system. Mining, and even agriculture (with labeling of ammonium nitrate like that used in the Oklahoma City bomb) would be affected. And in the case of gunpowder, which is often pre-blended at the factory to achieve specific performance characteristics, with each lot being distributed in small quantities to thousands of end users, the costs could be substantial. Taggant evidence alone can't identify one particular individual.
Finally, in response to a request by the U.S. Congress, the National Academy of Sciences is currently studying the types and use of taggants in explosives, with a final report due in February 1998. Whether the use of such technical evidence can make a difference for justice in a sensationalized criminal case is today a matter of substantial doubt, and that's a question no panel of distinguished scientists can resolve. If jurors lack a basic scientific background to evaluate the testimony of experts called upon to interpret the data, they may just ignore what they don't understand, and all of the best designed schemes for catching the bad guys will be for naught.
REFERENCES
Taggants in Explosives, Congress of the United States, Office of Technology Assessment, April 1980, SuDoc# Y3.T22/2:2Ex7 [This report, now rather outdated, is also available in a summary version with the SuDoc# Y3.T22/2:2Ex7/sum.]
Shanley, Agnes, ed., Harriston, Deborah with Roberts, Sandra, "The Unmistakable Fingerprint of Taggants," Chemical Engineering, v.103, p.35 (September 1996)
Wu, Corrina, "Tagged Out," Science News, v.150, pp.168-169 (September 14, 1996)
"Marking of products to establish identity and source," U.S. Patent #5,429,952 issued July 4, 1995 to Biocode, Inc.
Isotag LLC website at http://www.isotag.com/ [Warning: Isotag website disables "Back" button!]
National Academy of Sciences website at http://www.nas.edu/
Marking, Rendering Inert, and Licensing of Explosive Materials, by Committee on Marking, Rendering Inert, and Licensing of Explosive Materials, National Research Council, National Academy Press, ISBN 0-309-05990-9, (1998)
