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]-.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]-and [RDX + NO2]-.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.