1 INTRODUCTION
Ion mobility spectrometry (IMS) has been widely used for the detection of hazardous materials such as explosives, drugs, chemical warfare agents, and polycyclic aromatic hydrocarbons (PAHs); it involves the use of large laboratory instruments, portable analyzers, or on-site detectors.1-17 IMS is also one of the most widely used methods for detecting explosives in the field such as airport security checks. Various sampling methods for explosive detection via IMS have been reported.15,18-26 For gas-phase sampling, a direct gas inlet, gas chromatograph, and semi-permeable membrane have been investigated.20-22,26 The air sampling method can be conducted for the collection of explosive materials in the air by using a semi-permeable membrane upon air inhalation; however, it is applicable only when the concentration of explosive vapor is considerably high in the air. Vapor pressures of common explosives such as 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), and pentaerythritol tetranitrate (PETN) are very low of 5.50×10-6, 3.30×10-9, and 1.16×10-8 Torr at 25oC, respectively.27
Explosives with low vapor pressure can be vaporized by various methods including heating an explosive powder, nebulizing and heating an explosive solution, evaporating an explosive powder at room temperature for a long time, and using a vapor generator.28-32Mullen et al. studied explosive vapor transport efficiency using aqueous solutions of TNT and RDX by nebulizing and heating them at 130oC for the preparation of explosive vapor,28 while Li et al. prepared explosive vapor by placing a solid-state explosive powder in a flask at 25oC for 48 h after sealing.31 Vapor preparation by heating is not suitable for thermally labile chemicals; also, it is difficult to control the amount of vapor. Because vapor preparation at room temperature takes considerably long and the amount of vapor is low, it is not a suitable method for vaporizing explosives with low vapor pressure.
Explosive vapor prepared by heating can be detected through IMS.25,30 Collection and thermal desorption of explosive vapors using quartz and packed tubes was reported.33 However, these collection tubes need an additional thermal desorption device to evaporate the adsorbed explosives; moreover, this method suffers a risk of contamination when the tubes are reused. IMS has high explosive detection sensitivity and allows fast on-site detection of explosives. Hence, a simple and low-cost method for explosive vapor collection is required. Swab and smear matrices are typically used for collecting solid-state explosives for IMS detection.34-37 Furthermore, they are simple and affordable.
In this study, vapors of common explosives (TNT, RDX, and PETN) with low vapor pressure were artificially generated by spraying their solutions in acetone. Explosive molecules in aerosols produced by spraying are isolated by solvent evaporation. Acetone quickly evaporates even at room temperature, and evaporation is accelerated by spraying the solution. This explosive vapor preparation method enables control of the amount of explosive vapor. Artificial vapor was collected using various collection matrices, and the collected vapor was analyzed through IMS. Fifteen collection matrices comprising various substances and having different structures were employed. The differences in the explosive vapor collection efficiencies depending on the collection matrices were investigated. The influence of the arrangements (horizontal and vertical positions) of the collection matrix on the detection efficiency was also examined. The experimental results were explained on the basis of the differences in the collection matrix structures and the interactions between the matrix and explosive molecule. We believe that the developed method can be useful for testing the vapor detection efficiency of hazardous materials with low vapor pressure.