Trace explosives sampling for security applications (TESSA) study: Evaluation of procedures and methodology for contact sampling efficiency.

The detection of trace amounts of explosive materials is critical to the security at mass transit centers (e.g., airports and railway stations). In a typical screening process, a trap is used to probe a surface of interest to collect and transfer particulate residue to a detector for analysis. The collection of residues from the surface being probed is widely viewed as the limiting step in this process. A multi-institutional study was performed to establish a methodology for the evaluation of sampling media collection efficiencies. Dry deposited residues of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), C-4 (an RDX-based explosive), and pentaerythritol tetranitrate (PETN) were harvested from acrylonitrile butadiene styrene (ABS) plastic, ballistic nylon (NYL), and uncoated aluminum surfaces using muslin, Texwipe cotton, and stainless-steel mesh traps. Transfer and collection efficiencies of the sample media were calculated based on liquid chromatography-mass spectrometry analysis. Dry transfer efficiencies (DTE%) to all tested surfaces were greater than 75%, with transfer to ABS plastic being the lowest. Collection efficiency (CE%) varied significantly across the traps and the surfaces, yet some conclusions can be drawn; nylon had the lowest CE% for all cases (∼10%), and the stainless steel mesh had the lowest CE% for the evaluated traps (∼20%). Though the testing parameters have been standardized among the participants to establish a framework for an independent comparison of contact sampling media and surfaces, substantial variations in the DTE% and the CE% were observed, suggesting that other variables can affect contact sampling.

Aerodynamic resuspension and contact removal of energetic particles from smooth, rough, and fibrous surfaces.

Surface sampling for trace explosives residues is a critical step in the security screening in which microparticles are collected for subsequent chemical analysis. The current surface swabbing approach suffers from limited sampling area coverage, uncertainty in harvesting efficiencies, and user bias. Non-contact sampling has received interest due to its ability to interrogate large surface areas without the redeposition of the collected sample. However, the aerodynamic liberation of energetic particles from different types of substrates has not been parameterized or directly compared with the contact sampling methods. Here, we report aerodynamic resuspension rates of TNT, RDX, and HMX microparticles from smooth, rough, and fibrous surfaces. The resuspension thresholds are correlated to the boundary layer properties, i.e., wall shear stresses (τw = 50-500Pa). These rates are then compared to contact sampling for five commercial swabs using a standardized swabbing method. LC-MS analysis is used for the quantification of particle removal efficiencies. Contact sampling has an advantage over the low shear stress cases for particle liberation from the smooth surfaces. Aerodynamic particle resuspension rates increase with the wall shear stress. It shows better results for rough and fibrous surfaces than contact removal for tested analytes.

Tip-Enhanced Photoinduced Electron Transfer and Ionization on Vertical Silicon Nanowires.

Nanostructured semiconductors are one of the most potent candidates for matrix-free laser desorption/ionization mass spectrometric (LDI-MS) analysis of low-molecular-weight molecules. Herein, the enhanced photoinduced electron transfer and LDI on the tip of a vertical silicon nanowire (SiNW) array were investigated. Theoretical simulation and LDI detection of indigo and isatin molecules in negative ion mode revealed that the electric field can be enhanced on the tip end of SiNWs, thereby promoting the energy and electron transfer to the analytes adsorbed on the tip of SiNWs. On the basis of this finding, a tip-contact sampling method coupled with LDI-MS detection was established. In this strategy, the tip of SiNWs can be regarded as microextraction heads for the sampling of molecules when they come in contact with analytes. Impression of skin, tissue, and pericarp on the vertical SiNW array can effectively transfer endogenous metabolites or exogenous substances onto the tip. Upon laser irradiation, the adsorbed molecules on the SiNW tip can be efficiently ionized and detected in negative ion mode because of the tip-enhanced electron transfer and LDI effect. We believe this work may significantly expand the application of LDI-MS in various fields.