of a tagged explosive to the ultimate recovery of taggants by investigators at the site of a criminal bombing. The following analysis attempts qualitatively to cover the types and ranges of contamination that might be encountered and to suggest implications for law enforcement. It must be kept in mind that different types of explosives—e.g., cap-sensitive packaged explosives such as dynamite, as opposed to bulk fertilizer-grade AN—may be subject to distinct types or sources of contamination. It is presumed here that taggants have been supplied to the explosives manufacturers in uncontaminated form, with accurate records of codes employed and of schedules of taggant incorporation.
During the manufacturing process, improperly cleaned equipment may contain the residue of a previous batch of explosive that had its own, differently coded taggant. An explosive produced with contaminated equipment would thus contain some fraction, x, of differently coded taggant particles, while the remaining fraction 1 – x would be correct. The fraction x would depend strongly on the details of the particular manufacturing equipment and processes (and therefore on the specific product involved), on the frequency with which the taggant code was changed, and on the number of batches produced previously under the same code. As a result of these variations, x could reasonably be estimated to range anywhere from virtually zero to approximately 0.02. Stringent cleaning protocols for the processing equipment could hold this number close to an absolute minimum, but at some cost burden to the manufacturer and legitimate consumer.
In packaged explosives, taggant purity is unlikely to be compromised to any significant extent during transportation and distribution. By contrast, these activities might be substantial sources of contamination for unpackaged bulk products. For example, creating and moving large heaps of AN as well as loading and unloading it from trucks and river barges would surely reduce taggant purity. Also, the comingling of products from different manufacturers, an occasional practice in the handling of bulk AN, might also reduce taggant specificity. If y denotes the fraction of contamination introduced during the transportation and distribution of an explosive, the reduction of the purity of the taggant as it existed at the factory output stage can be signified by 1 – y. Clearly, this factor will depend on the type of explosive involved and on the presence or absence of anticontamination measures taken during transportation and distribution, especially for bulk products such as AN. The factor 1 – y could be virtually 1 in favorable circumstances that might realistically apply to packaged explosives, but the committee estimates that a combination of contaminating effects especially relevant to bulk AN might at least temporarily reduce it to 0.8 (y = 0.2).
At a bomb site, environmental contamination potentially introduces yet another source of error in identifying the "correct" taggant. For example, taggant particles driven outward from an explosion could become embedded in walls of a