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.