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.