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Synthesis and caracterization of molecularly imprinted sorbents for the selective extraction of explosive residues and for the development of on-site procedure using portable devices
For the last twenty years, people have suffered of numerous deadly terrorist attacks. Today, it remains hard to identify precisely the used explosives considering the complex matrices (motor oil, human blood…) of the post-blast samples and the explosive residues contained at the trace level. For this, a sample clean-up is required to clean the sample and preconcentrate the explosives before chromatography analysis, in order to identify and quantify them easily. Solid phase extraction has become the method of choice for the treatment of such complex samples. Various extracting sorbents are available but coextraction of interfering compounds can be observed. Thus, polymeric sorbents called MIP (molecularly imprinted polymers) developing a retention based on a molecular recognition mechanism have been suggested as sorbent for the selective extraction of the nitroaromatics which are some of the most commonly used explosives.
First, the sorbents were synthesized by radical polymerization. Nitro groups of targeted compounds didn’t allow the formation of sufficiently strong polar interactions to obtain specific cavities during the synthesis. Only the use of hydrophobic and/or π-π interactions with the styrene as monomer allowed the formation of selective cavities towards 2,4-dinitrotoluene (2,4-DNT) and 2,4,6-trinitrotoluene (2,4,6-TNT) to obtain a weak selectivity during the extraction. Thus another alternative, a sol-gel synthesis, was considered in order to retain selectively nitroaromatic compounds from complex samples. Those silica sorbents were synthesized in aqueous media. The formation of specific cavities was based on strong interactions between the template (2,4-DNT) and the monomer (phenyltrimethoxysilane) in order to develop hydrophobic and π-π interactions. Different sorbents were synthesized, with three different molar ratios between template / monomer / cross-linker and characterized by the elution profile and the estimation of the capacity.
Finally an imprinted silica sorbent allowed for the first time according to our knowledge, the simultaneous and selective extraction of four nitroaromatic compounds (2,4-DNT, 2,4,6-TNT, 2,6-DNT and tetryl) with extraction recoveries greater than 88 ± 4 % (n = 3). These results were confirmed by extraction in real media with real matrix such as motor oil and blood and comparison with a conventional sorbent.
Given the great potential of this kind of sorbent for the selective extraction of nitroaromatics, miniaturization of all the analytical tools was investigated, in order to develop a portable device for on-site procedure. For this purpose, an on-line extraction needing a small amount of sorbent was carried out. Then, an electrochemical detection method of 2,4,6-TNT on imprinted electrodes has been successfully developed. By applying the sample purification on MIS before the electrochemical detection, this device is the first step towards a on-site procedure.