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.