September 9, 2009 By Blake Harris
Photo: SRM 2905, Trace Particulate Explosive Simulants, consists of four different test substances designed to simulate trace residues of C-4 plastic explosives and TNT. Seen through a pair of specially filtered glasses under a blue crime-scene light, a spot of the fluorescently tagged SRM is plainly visible (lower middle) on the test paper. (NIST)
Anyone who travels frequently by air has probably been checked for explosives going through airport security - either through a prototype walkthrough portal or tabletop-type trace explosive detectors. As well, firefighters and police officers use such detection systems to evaluate suspicious packages and customs inspectors sometimes check international cargo shipments.
The object of such screenings, of course, is to find explosive materials or persons who have been in contact with them. To do this, detectors need to effectively collect residue particles that result from handling materials that might be used to fabricate a bomb and then evaluate the explosives content. For example, when operating the tabletop device, security personnel use a piece of material to swab packages and bags for explosive residues. The security officer then places the swab in a tabletop device that heats the material, separating any chemical residues that may have been absorbed.
Like other sensitive instruments, these machines need well-defined calibration standards to ensure that they are working properly. The problems is, according to NIST chemist William MacCrehan, the calibration materials that the vendors of these machines provide are typically of unknown quality. "These detectors need to be reliable and precise enough to detect particles that weigh as little as a few billionths of a gram," explained MacCrehan in a prepared statement.
To solve this problem, the National Institute of Standards and Technology (NIST), with support from the Department of Homeland Security, has developed a new certified reference material, Standard Reference Material (SRM) 2905, Trace Particulate Explosives.
SRM 2905 consists of four different test substances designed to simulate trace residues of C-4 plastic explosives and TNT. The substances themselves consist of inert solid particles about 20 to 30 microns in diameter. The particles have been coated with explosive materials and a florescent tag, which enables the material to be seen using specially filtered optics or glasses. Although the particles are coated with explosive material, MacCrehan says they are incapable of exploding on their own and are completely safe to handle.
"We created this SRM to provide manufacturers and operators with high quality, independently generated and validated reference test materials to enable better designs and reduce the number of false positives and negatives," MacCrehan added.
This release is part of a larger, ongoing project to develop other wet and dry materials that simulate SEMTEX, gunpowder and peroxide-type explosives. According to MacCrehan, efforts also are underway to develop reference materials to help train bomb-sniffing dogs.