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 cm2, 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 < 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.