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 × 64 = 5760 cm2, and the size of the collection matrix was 5 × 8 = 40 cm2. Thus, for the horizontal position, each collection matrix covered 0.7% of the hood base. If 20 µg 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 µg 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.