Explosive Contamination from Substrate Surfaces: Differences and Similarities in Contamination Techniques Using RDX and C-4

Abstract

Explosive trace detection equipment has been deployed to airports for more than a decade. During this time, the need for standardized procedures and calibrated trace amounts for ensuring that the systems are operating properly and detecting the correct explosive has been apparent but a standard representative of a fingerprint has been elusive. Standards are also necessary to evaluate instrumentation in the laboratories during development and prior to deployment to determine sample throughput, probability of detection, false positive/negative rates, ease of use by operator, mechanical and/or software problems that may be encountered, and other pertinent parameters that would result in the equipment being unusable during field operations. Since many laboratories do not have access to nor are allowed to handle explosives, the equipment is tested using techniques aimed at simulating the actual explosives fingerprint. This laboratory study focused on examining the similarities and differences in three different surface contamination techniques that are used to performance test explosive trace detection equipment in an attempt to determine how effective the techniques are at replicating actual field samples and to offer scenarios where each contamination technique is applicable. The three techniques used were dry transfer deposition of standard solutions using the Transportation Security Laboratory's (TSL) patented dry transfer techniques (US patent 6470730), direct deposition of explosive standards onto substrates, and fingerprinting of actual explosives onto substrates. RDX was deposited on the surface of one of five substrates using one of the three different deposition techniques. The process was repeated for each substrate type using each contamination technique. The substrate types used were: 50% cotton/50% polyester as found in T-shirts, C. J. Miller (100% cotton with a smooth surface such as that found in a cotton dress shirt, 100% cotton on a rough surface such as that found on canvas or denim, suede leather such as might be found on jackets, purses, or shoes, and painted metal obtained from a car hood at a junk yard. The samples were not pre-cleaned prior to testing and contained sizing agents, and in the case of the metal, oil and dirt. The substrates were photographed using a Zeiss Discover V12 stereoscope with Axiocam ICc1 3 megapixel digital camera to determine the difference in the crystalline structure and surface contamination in an attempt to determine differences and similarities associated with current contamination deposition techniques. Some samples were analyzed using scanning electron microscopy (SEM) and some were extracted and analyzed with high performance liquid chromatography (HPLC) or gas chromatography with an electron capture detector (GC-ECD) to quantify the data.

Advances in sublimation studies for particles of explosives

“…The nature or "ground truth" of the explosives in latent fingerprints has been examined by a number of researchers 1,6,7 revealing the presence of different-sized explosive particles adhered to surfaces contacted. For examples reported in the literature, the amount of explosives material found in a single C-4 explosives fingerprint range from about ~ 1 -10 g 4 .…”

Polymorphic Phase Control of RDX-Based Explosives

“…1,8,11,12 Theoretical and experimental Raman spectra of RDX show multiple characteristic bands that distinguish the a-phase from the metastable b-polymorph. 1,[6][7][8][9][10][11][12][13] For example, when RDX is in the b-conformation, a decrease in the number of vibrational modes and changes in the spectral signature such as shifting and disappearance of bands is observed when compared against a-RDX. 14 The symmetrical ring-breathing mode is an example of one band that undergoes a shift and its position has been used as an indication of ring strain.…”

A method to control the polymorphic phase for RDX-based trace standards

“…Theoretical and experimental Raman spectra of 3,5-trinitro-1,3,5-triazine (RDX) show multiple characteristic bands that distinguish the α phase which is the bulk, thermodynamically stable (i.e., lowest energy) form from the metastable β-polymorph (a conformational polymorph α-RDX) [1][2][3][4][5][6][7][8][9] . For example, when RDX is in the β-conformation a decrease in the number of vibrational modes and changes in spectral signature such as shifting and disappearance of bands is observed when compared against α-RDX 10 .…”