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.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 × 8 cm2. 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.