Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

Abstract

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common procedure for collecting traces of explosives, and standardized measurements of collection efficiency are needed to evaluate and optimize sampling protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Three critical factors (the applied force, travel distance, and travel speed) are controlled using an automated device. Test surfaces are chosen based on similarity to the screening environment, and the wipes can be made from any material considered for use in wipe-sampling. Particle samples of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) are applied in a fixed location on the surface using a dry-transfer technique. The particle samples, recently developed to simulate residues made after handling explosives, are produced by inkjet printing of RDX solutions onto polytetrafluoroethylene (PTFE) substrates. Collection efficiency is measured by extracting collected explosive from the wipe, and then related to critical sampling factors and the selection of wipe material and test surface. These measurements are meant to guide the development of sampling protocols at screening venues, where speed and throughput are primary considerations.

A new wipe-sampling instrument for measuring the collection efficiency of trace explosives residues

“…For the remaining experiments, the 660 g applied load was used to compare to recent literature. 45,46…”

“…Recent work by Verkouteren et al 45,46 developed protocols for preparing trace explosive samples for dry-transfer to a test surface and measuring collection by wipe-sampling from the surface. These protocols were followed for this work.…”

“…Until recently, a standardized method for measuring the collection efficiency (CE%) of explosive particles after interrogating a test surface with a wipe had not been established. Verkouteren et al 46 addressed this by describing key factors to consider when designing wipe-sampling experiments. The capabilities of the wipe-sampling instrument and features to consider when choosing wipes and test surfaces to investigate were discussed.…”

“…Previous wipe-sampling efficiency studies 36,46,48,49 for trace explosives detection have typically used a TL-slip/peel tester to allow for programable and reproducible speeds and wipe distances. The TL-slip/peel tester is a useful instrument; however, it has its limitations.…”

“…Another potential issue with the TL-slip/peel tester, is the flexible wire that holds the wipe mount in place. If the wire is not properly straightened, 46 the wipe will not have a smooth motion as the surface is translated underneath it. This zig-zagging movement could also negatively impact the CE%.…”