New particle-based trace explosive test material produced by drop-on-demand inkjet printing for quantitative wipe-sampling studies

A new approach is described to prepare particle standards of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) for use in quantitative wipe-sampling studies. Screening activities to detect traces of explosives are heavily focused on wipe collection from surfaces such as fabrics, metals, and plastics with speed and efficacy as the goals. Studies to improve the sampling protocols, including development of improved wipe materials and selection of best surfaces, depend upon the availability of test materials that mimic particles in fingerprints made after handling explosives. RDX particle arrays were prepared by drop-on-demand (DOD) inkjet printing onto polytetrafluoroethylene (PTFE) substrates and then transferred by rubbing onto target surfaces. The particle size distributions in the transferred deposits approximately match the distributions measured from fingerprints (size range of 1 to 40 mm) for equivalent total masses of RDX, and the transfer is better than 95% efficient to three realistic surfaces; woven nylon, synthetic leather, and brushed steel. DOD inkjet printing can be tailored to produce the desired particle sizes by changing the total deposit mass at each point in the array. Deposits will not transfer efficiently to surfaces smoother than the PTFE transfer substrate. Wipe-sampling studies conducted with the new test materials at a fixed force and for a specific sampling wipe show that the sampling efficiency from woven nylon fabric (e.g., luggage) is 17 +/- 4% whereas the efficiency from acrylonitrile butadiene styrene (ABS) plastic (e.g., luggage handles) under the same conditions is 37 +/- 6%.