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\begin{document}

\title{Test method for vapor collection and ion mobility detection of
explosives with low vapor pressure}



\author[1]{Sung-Seen Choi}%
\author[1]{Chae Eun  Son}%
\author[1]{He-Ryun  Choi}%
\affil[1]{Sejong University}%


\vspace{-1em}



  
  \date{June 16, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Rationale: IMS has been widely used for the on-site detection of
explosives. Air sampling method is applicable only when the
concentration of explosive vapor is considerably high in the air, but
vapor pressures of common explosives such as TNT, RDX, and PETN are very
low. A test method for analyzing the vapor detection efficiency of
explosives with low vapor pressure via IMS was developed using
artificial vapor and collection matrices. Methods: Artificial explosive
vapor was produced by spraying an explosive solution in acetone. Fifteen
collection matrices of various materials with woven or nonwoven
structure were tested. Two arrangements of horizontal and vertical
positions of the collection matrices were employed. Explosive vapor
collected in the matrix was analyzed through IMS. Results: Only three
collection matrices of stainless steel mesh (SSM),
polytetrafluoroethylene sheet (PFS), and lens cleansing paper (LCP)
showed the TNT and/or RDX ion peaks at explosive vapor concentration of
49 ng/L. There was no collection matrix to detect PETN vapor at lower
than 49 ng/L. For the PFS, TNT and RDX were detected at 49 ng/L vapor
concentration. For the LCP, TNT and RDX were detected at vapor
concentrations of 14 and 49 ng/L, irrespectively. Conclusions: The
difference in the explosive vapor detection efficiencies could be
explained by the adsorption and desorption capabilities of the
collection matrices. The proposed method can be used for evaluating the
vapor detection efficiency of hazardous materials with low vapor
pressure.%
\end{abstract}%



\sloppy


\textbf{Test method for vapor collection and ion mobility detection of
explosives with low vapor pressure}

\textbf{Chae Eun Son, He-Ryun Choi, and Sung-Seen Choi*}

\emph{Department of Chemistry, Sejong University, 209 Neungdong-ro,
Gwangjin-gu, Seoul 05006, Republic of Korea}

* Correspondence should be addressed.

E-mail: sschoi@sejong.ac.kr

\textbf{Rationale:} IMS has been widely used for the on-site detection
of explosives. Air sampling method is applicable only when the
concentration of explosive vapor is considerably high in the air, but
vapor pressures of common explosives such as TNT, RDX, and PETN are very
low. A test method for analyzing the vapor detection efficiency of
explosives with low vapor pressure via IMS was developed using
artificial vapor and collection matrices.

\textbf{Methods:} Artificial explosive vapor was produced by spraying an
explosive solution in acetone. Fifteen collection matrices of various
materials with woven or nonwoven structure were tested. Two arrangements
of horizontal and vertical positions of the collection matrices were
employed. Explosive vapor collected in the matrix was analyzed through
IMS.

\textbf{Results:} Only three collection matrices of stainless steel mesh
(SSM), polytetrafluoroethylene sheet (PFS), and lens cleansing paper
(LCP) showed the TNT and/or RDX ion peaks at explosive vapor
concentration of 49 ng/L. There was no collection matrix to detect PETN
vapor at lower than 49 ng/L. For the PFS, TNT and RDX were detected at
49 ng/L vapor concentration. For the LCP, TNT and RDX were detected at
vapor concentrations of 14 and 49 ng/L, irrespectively.

\textbf{Conclusions:} The difference in the explosive vapor detection
efficiencies could be explained by the adsorption and desorption
capabilities of the collection matrices. The proposed method can be used
for evaluating the vapor detection efficiency of hazardous materials
with low vapor pressure.

\textbf{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.\textsuperscript{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.\textsuperscript{15,18-26} For gas-phase sampling, a
direct gas inlet, gas chromatograph, and semi-permeable membrane have
been investigated.\textsuperscript{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\selectlanguage{ngerman}×10\textsuperscript{-6}, 3.30×10\textsuperscript{-9}, and
1.16×10\textsuperscript{-8} Torr at 25\textsuperscript{o}C,
respectively.\textsuperscript{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.\textsuperscript{28-32}Mullen et al. studied explosive vapor
transport efficiency using aqueous solutions of TNT and RDX by
nebulizing and heating them at 130\textsuperscript{o}C for the
preparation of explosive vapor,\textsuperscript{28} while Li et al.
prepared explosive vapor by placing a solid-state explosive powder in a
flask at 25\textsuperscript{o}C for 48 h after
sealing.\textsuperscript{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.\textsuperscript{25,30} Collection and thermal desorption of
explosive vapors using quartz and packed tubes was
reported.\textsuperscript{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.\textsuperscript{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.

\textbf{2 METHODS}

\textbf{2.1 Materials}

TNT, RDX, and PETN were supplied from Hanwha Co. (Republic of Korea).
Acetone was purchased from Aldrich Chemical Co. (USA). Stainless steel
mesh (SSM) and polytetrafluoroethylene (PTFE) sheet (PFS) were supplied
from Newone S\&T (Republic of Korea). Cotton fabric (COF) was purchased
from Chungoa Co. (Republic of Korea), lens cleansing paper (LCP) was
purchased from Sargent-Welch Scientific Co. (USA), polypropylene (PP)
nonwoven (PPN) was purchased from Thesoop Co. (Republic of Korea),
acrylate fabric (APF), poly(ethylene terephthalate) (PET) fabric (PTF),
and polyamide (PA) fabric (PAF) were purchased from Dawon Co. (Republic
of Korea), carbon nonwoven (CFN) was purchased from GNT filter Co.
(Republic of Korea), carbon plain/twilled fabrics (CPF and CTF,
respectively) and carbon fabrics (CF720 and CF730) (CF2 and CF3,
respectively) were purchased from Hankuk Carbon Co. (Republic of Korea),
filter paper (FPP) was purchased from Hyundai Micro Co. (Republic of
Korea), and coffee filter (CFP) was purchased from Thomas \& Green Co.
(Singapore). Only SSM is made of metal and has mesh structure.
Collection matrices with woven fabric structures are PFS, COF, APF, PTF,
PAF, CPF, CTF, CF2, and CF3, while those with nonwoven structures are
LCP, PPN, CFN, FPP, and CFP (Table S1 and Figure S1; Supplementary
Information).

\textbf{2.2 Preparation of artificial explosive vapor and vapor
collection experiment}

Each explosive was dissolved in a volatile solvent of acetone, and
explosive solutions of 4.17 \selectlanguage{greek}μ\selectlanguage{english}g/mL were prepared. Experiments were
carried out using a DLH-010 fume hood (410 L) of Jeiotech Co. (Republic
of Korea). The dimension of the collection matrix was 5 \selectlanguage{ngerman}× 8
cm\textsuperscript{2}. Explosive vapor was artificially produced by
spraying the explosive solution. A total of 5, 10, and 20 \selectlanguage{greek}μ\selectlanguage{english}g of the
explosive were introduced into the hood by spraying 1.2, 2.4, and 4.8 mL
of the explosive solution, respectively. These values correspond to the
explosive vapor concentrations of 12, 24, and 49 ng/L, respectively, in
the hood. The experimental procedure was as follows: (1) two sets of the
matrices were arranged at the bottom part of the hood, (2) the explosive
solution was sprayed at the top part of the hood, (3) the explosive
vapor was collected for 30 sec, and (4) IMS analysis was carried out
with the collection matrix. The generation of artificial explosive vapor
and the vapor deposition in the collection matrix are described in
Figure 1. Aerosols were formed by spraying the explosive solution, which
leads to the acceleration of acetone evaporation.

Two arrangement types of the horizontal and vertical positions of the
collection matrices were employed as shown in Figures 2 and 3,
respectively. The base side area of the hood was 90 \selectlanguage{ngerman}× 64
cm\textsuperscript{2}. For the horizontal arrangement, two sets of 15
collection matrices were placed at 12 cm from the wall. For the vertical
arrangement, two sets of 15 collection matrices were placed at 14 and 15
cm from the wall. Two sets of collection matrices were located at
different sites to avoid positional effects.

\textbf{2.3 IMS analysis}

Portable IMS equipped with corona discharge ionization source, IMS of
Newone S\&T (Republic of Korea), was used. The analysis conditions of
IMS were as follows: temperature of the sample inlet part was
150\textsuperscript{o}C, temperature of the drift tube was
100\textsuperscript{o}C, the electric field was 200 V/cm, and the ion
drift tube was 13.2 cm. The IMS analysis was performed in the negative
ion mode. The same experiments were performed 5 times and it was
determined to detect the explosive when detecting at least four of the
five cases.

Detection limits of the solid-state explosives according to the
collection matrices were analyzed. The explosive solutions of 0.1 - 500
ng/\selectlanguage{greek}µ\selectlanguage{english}L were prepared. The explosive solution of 1.0 \selectlanguage{greek}µ\selectlanguage{english}L was dropped on the
collection matrix and the solvent was thoroughly evaporated to obtain
solid-state explosive. The matrix was introduced into the sample inlet
part of IMS and the IMS analysis was carried out.

\textbf{3 Results and Discussion}

\textbf{3.1 Explosive vapor collection efficiency depending on the
collection matrix}

The experimental results are summarized in Table 1. Among the 15
collection matrices, only three matrices of the SSM, PFS, and LCP showed
the TNT and/or RDX ion peaks in the IMS spectra at the explosive vapor
concentration lower than 49 ng/L. There was no collection matrix to
detect PETN at a vapor concentration lower than 49 ng/L. For the SSM,
only TNT at 49 ng/L vapor concentration was detected for the horizontal
position. For the PFS, TNT and RDX were detected at 49 ng/L vapor
concentration irrespective of the arrangement types. For the LCP, TNT
was detected at 24 ng/L vapor concentration and RDX was observed at 49
ng/L vapor concentration irrespective of the arrangements.

The base side area of the hood was 90 \selectlanguage{ngerman}× 64 = 5760 cm\textsuperscript{2},
and the size of the collection matrix was 5 × 8 = 40
cm\textsuperscript{2}. Thus, for the horizontal position, each
collection matrix covered 0.7\% of the hood base. If 20 \selectlanguage{greek}µ\selectlanguage{english}g artificial
vapor completely and evenly spreads down to the hood base without any
loss, the maximum amount adsorbed on each collection matrix will be 140
ng. However, considerable amount of explosive vapor cannot reach the
hood base, because some explosive vapor will be adsorbed on the hood
wall during the spraying of solution and spreading of vapor. Considering
the adsorption of explosive vapor on the hood wall, a reasonable amount
of explosive vapor adsorbed on each collection matrix should be much
less than 140 ng. When 10 \selectlanguage{greek}µ\selectlanguage{english}g vapor is introduced into the hood, the
amount of explosive vapor adsorbed on each collection matrix should be
much less than 70 ng. Considering this situation, the corrected
detection limits of explosive vapors deposited on the collection
matrices are summarized in Table 2.

\textbf{3.2 Comparison of detection efficiencies of collected explosive
vapor and solid-state explosive samples}

The corrected detection limits of explosive vapors were compared to the
detection limits of solid-state explosives. The solid-state explosive
samples were prepared by dropping the explosive solution on the
collection matrix and evaporating the solvent. The detection limits of
solid-state explosives are listed in Table 3. The detection limits of
solid-state explosives were significantly lower than those of the vapor
samples. This is because of the difference in the distributions of the
explosive samples in the matrix. For the solid-state explosives, the
sample spot size is typically less than 1 cm\textsuperscript{2}, whereas
for the collected explosive vapor the explosive molecules are evenly
distributed over the entire matrix (Figure S2; Supplementary
Information). Because the diameter of the inlet hole in the IMS
instrument is less than 1 cm, a large amount of explosives adsorbed on
the matrix cannot be sufficiently introduced into the ionization part.
Thus, the inlet efficiency for the vapor-collected sample will be
significantly lower than that for the solid-state explosive sample.

The LCP exhibited the best detection limit for the solid-state
explosives, followed by the SSM. However, the detection limit of SSM for
the explosive vapor was higher than that of PFS. This is because the SSM
has a mesh structure, which limits the absolute area available to
collect explosive vapor. Although the detection limits of solid-state
TNT were less than 10 ng except for the PTF and CFN, the TNT vapor of 49
ng/L (the corrected amount \textless{} 140 ng) was not detected except
for the SSM, PFS, and LCP. This can be explained by the thickness and
bundle structure of the fine filaments. The matrices with woven fabric
structures, such as the COF, APF, PTF, PAF, CFP, CF2, and CF3, are
composed of bundles of fine filaments, while those with nonwoven
structures, such as the PPN (0.33 mm), CFN (3.49 mm), FPP (0.14 mm), and
CFP (0.11 mm), are relatively thick (Table S1 and Figure S1;
Supplementary Information). For the woven fabric matrices, explosive
vapor can penetrate into the inner part of the fine filament bundles,
thereby reducing the desorption efficiency of the explosive molecules
collected in the inner part. Although the PFS is a woven fabric, it is
chemically inert and the filaments are very close to each other, so the
explosive vapors may not deeply penetrate into the matrix. For the thick
nonwoven matrices, explosive vapor can penetrate into the inner part of
the matrix, thereby reducing the desorption efficiency.

\textbf{3.3 Characteristics of IMS spectra of the collected explosive
vapor samples}

Figure 4 shows the IMS spectra of TNT vapors (24 ng/L) collected in the
LCP. The TNT ion was detected for both the horizontal and vertical
arrangements. The TNT ion peak is {[}TNT --
H{]}\textsuperscript{-}.\textsuperscript{7,28-30} For the horizontal
arrangement, the TNT ion peak was more intensively observed at 5 -- 6
sec after the sample inlet. For the vertical arrangement, the TNT ion
peak was more intensively detected at 5 -- 6 sec after the sample inlet
and it was observed until 10 sec. Detection sensitivity for the vertical
arrangement was better than that for the horizontal arrangement, which
was unexpected; however, it can be attributed to the flexibility and
adsorption ability of the thin LCP (0.03 mm thickness). The LCP arranged
with the horizontal position is fixed at the bottom and explosive vapor
is adsorbed on the upper side, whereas for the vertical arrangement it
is movable and explosive vapor is adsorbed on the both sides.

In the IMS spectra of TNT vapors (49 ng/L) adsorbed on the SSM, the TNT
ion was detected at 36.3 ms for the horizontal arrangement, but it was
not observed for the vertical arrangement (Figure S3; Supplementary
Information). The TNT ion peak was more intensively observed at 5 -- 6
sec after the sample inlet. In the IMS spectra of RDX vapors (24 ng/L)
adsorbed on the PFS, the RDX ion was detected at approximately 38 ms for
both the horizontal and vertical arrangements, but its intensity was
weak (Figure S4; Supplementary Information). The intensity of the RDX
ion peak was greater at longer delay times, which may be attributed to
low volatility of RDX.

Figure 5 shows the IMS spectra of RDX vapors (49 ng/L) adsorbed on the
LCP. For the horizontal arrangement, the RDX ion was detected at the
delay time of 1 -- 3 sec and two ion peaks were observed after 5 sec.
The two RDX-related ions correspond to {[}RDX +
Cl{]}\textsuperscript{-}and {[}RDX +
NO\textsubscript{2}{]}\textsuperscript{-}.\textsuperscript{28-31} For
the vertical arrangement, the two RDX-related ions were clearly detected
at 37.3 and 39.5 ms after 5 sec. Similar to the TNT case, the detection
sensitivity for the vertical arrangement of the LCP was greater than
that for the horizontal arrangement, which is also because of the
flexibility and adsorption ability of the LCP as discussed above.

\textbf{3.4 Factors influencing the explosive vapor detection
efficiency}

The PETN vapor was not detected up to a vapor concentration of 49 ng/L
for any of the collection matrices. This cannot be explained by the low
vapor pressure of PETN because the vapor pressure of PETN is greater
than that of RDX.\textsuperscript{27} This can be attributed to the
structural flexibility and the number of the nitrate groups of PETN
molecule. The nitrite groups of TNT molecule are fixed in the rigid
benzene ring, whereas those of RDX molecule are bonded to the
six-membered ring of flexible methylene. The nitrate groups of PETN
molecule are bonded to flexible alkyl chain. The nitrate group is more
flexible than the nitrite group. The energy-minimized structures of TNT,
RDX, and PETN molecules are shown in Figure 6. PETN contains four
nitrate groups, while TNT and RDX contain three nitrite groups. Since
the nitrate groups better interact freely and strongly with the polymers
of the collection matrix than the nitrite ones, the desorption
efficiency of the adsorbed explosives will be reduced.

Factors influencing the detection efficiency of explosive vapor can be
divided into two categories of the adsorption and desorption
capabilities. The factors affecting the adsorption capability of the
collection matrix are area, material, and structure. All of the
collection matrices have the same size of 5 \selectlanguage{ngerman}× 8 cm\textsuperscript{2}.
However, the real adsorption area of SSM is much smaller than those of
the others because of its mesh structure. The types of materials of the
collection matrices can also influence the adsorption capability of
explosive vapor. If a collection matrix is chemically more interactive
with explosive vapor, the adsorption capability is expected to improve.
Since the explosives are relatively polar, the collection matrix made of
medium polar material may improve the adsorption capability.

The influencing factors related to the desorption capability of the
collection matrix are chemical interactions, thermal conductivity,
thickness, and structure. The desorption capability is compromised
because of the increasing interactions between the explosive molecule
and collection matrix. Since PETN might have stronger interactions with
the collection matrix compared to the other explosives as discussed
above, its desorption capability is expected to be poorer. The thermal
conductivity of the collection matrix affects the desorption capability
because the detection sensitivity depends on the amount of explosive
molecules desorbed at a certain time. A collection matrix with good
thermal conductivity can induce a more rapid and efficient evaporation
of the explosives, thereby improving the detection sensitivity. Since
explosive vapors can penetrate into the inner part of the collection
matrix, a thicker matrix is less favorable for desorbing the explosive
molecules compared to a thinner matrix. The bundle structure of fine
filaments can improve the adsorption capability of explosive vapor, but
will prevent thermal conduction. Thus, collection matrices with a bundle
structure would exhibit a poor detection efficiency.

\textbf{4 CONCLUSIONS}

A test method for vapor collection efficiency and IMS detection of
explosives with low vapor pressure such as TNT, RDX, and PETN was
developed using artificial explosive vapor and collection matrices.
Explosive vapor was generated by spraying the explosive solution in
acetone, and the collected vapor was detected using IMS. Of the 15
collection matrices, only the SSM, PFS, and LCP showed the TNT and/or
RDX ion peaks in the IMS spectra at explosive vapor concentration of
lower than 49 ng/L. PETN was not detected up to a vapor concentration of
49 ng/L because of the low desorption capability. For the SSM, only TNT
at 49 ng/L of vapor concentration was detected for the horizontal
arrangement. For the PFS, 49 ng/L of TNT and RDX vapors were observed in
the IMS spectra. For the LCP, TNT was detected at 24 ng/L of vapor
concentration and RDX was observed at 49 ng/L of vapor concentration.
The detection limits of explosive vapors were significantly poorer than
those for solid-state explosives owing to the difference in the
introduction efficiencies. The adsorption and desorption capabilities
could be factors that influence the explosive vapor detection
efficiency. The factors affecting the adsorption capability of the
collection matrix are area, material, and structure. The contributing
factors related to the desorption capability are chemical interactions,
thermal conductivity, thickness, and structure. The test method
developed in this study can be used for evaluating vapor collection
efficiency and IMS detection of hazardous materials with low vapor
pressure and may be applied to on-site trace chemical vapor detection.

\textbf{Acknowledgements}

This research was supported by Korea Institute of Marine Science \&
Technology Promotion (KIMST) funded by the Ministry of Oceans and
Fisheries, Korea (20200611).

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\textbf{Table 1 Detection of explosive vapors collected in the
collection matrices.}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Explosive concentration (ng/L) & Collection matrix (Horizontal/Vertical)
& Collection matrix (Horizontal/Vertical) & Collection matrix
(Horizontal/Vertical)\tabularnewline
\midrule
\endhead
& SSM & PFS & LCP\tabularnewline
TNT (12) & No/No & No/No & No/No\tabularnewline
TNT (24) & No/No & No/No & Yes/Yes\tabularnewline
TNT (49) & Yes/No & Yes/Yes & Yes/Yes\tabularnewline
RDX (24) & No/No & No/No & No/No\tabularnewline
RDX (49) & No/No & Yes/Yes & Yes/Yes\tabularnewline
PETN (49) & No/No & No/No & No/No\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 2 Corrected detection limits of the explosive vapors
collected in the collection matrices (ng).}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Explosive & Collection matrix & Collection matrix & Collection
matrix\tabularnewline
\midrule
\endhead
& SSM & PFS & LCP\tabularnewline
& Horizontal position & Horizontal position & Horizontal
position\tabularnewline
TNT & \textless{} 140 & \textless{} 140 & \textless{} 70\tabularnewline
RDX & \textgreater{} 140 & \textless{} 140 & \textless{}
140\tabularnewline
PETN & \textgreater{} 140 & \textgreater{} 140 & \textgreater{}
140\tabularnewline
& Vertical position & Vertical position & Vertical
position\tabularnewline
TNT & \textgreater{} 140 & \textless{} 140 & \textless{}
70\tabularnewline
RDX & \textgreater{} 140 & \textless{} 140 & \textless{}
140\tabularnewline
PETN & \textgreater{} 140 & \textgreater{} 140 & \textgreater{}
140\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 3 Detection limits of the solid-state explosives prepared
by the solution spot according to the collection matrices (ng).}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Collection matrix & TNT & RDX & PETN\tabularnewline
\midrule
\endhead
SSM & 2 & 5 & 5\tabularnewline
PFS & 5 & 20 & 20\tabularnewline
COF & 5 & 20 & 20\tabularnewline
LCP & 0.2 & 2 & 2\tabularnewline
PPN & 5 & 10 & 20\tabularnewline
APF & 7 & 70 & 70\tabularnewline
PTF & 20 & 70 & 70\tabularnewline
PAF & 5 & 50 & 50\tabularnewline
CFN & 500 & \textgreater{} 500 & \textgreater{} 500\tabularnewline
CPF & 7 & 50 & 50\tabularnewline
CTF & 7 & 50 & 50\tabularnewline
CF2 & 7 & 50 & 50\tabularnewline
CF3 & 7 & 50 & 50\tabularnewline
FPP & 5 & 20 & 20\tabularnewline
CFP & 5 & 20 & 20\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{FIGURE 1} Generation of artificial explosive vapor by spraying
the explosive solution and the vapor deposition in a collection matrix.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{FIGURE 2} Horizontal arrangement experiment of the collection
matrices. The numbers are the sample codes marked in Figure 1.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{FIGURE 3} Vertical arrangement experiment of the collection
matrices. The numbers are the sample codes marked in Figure 1.

\textbf{FIGURE 4} IMS spectra of the TNT vapors collected in the LCP
matrix arranged with horizontal (a) vertical and (b) positions. The
amount of explosive was 10 \selectlanguage{greek}µ\selectlanguage{english}g.

\textbf{FIGURE 5} IMS spectra of the RDX vapors collected in the LCP
matrix arranged with horizontal (a) vertical and (b) positions. The
amount of explosive was 20 \selectlanguage{greek}µ\selectlanguage{english}g.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{FIGURE 6}Energy-minimized structures of TNT, RDX, and PETN.

\selectlanguage{english}
\FloatBarrier
\end{document}

