Direct coupling of active and passive samplings of organics with microwave assisted thermal desorption as an innovative solvent-free method

Thermal desorption has been developed the past years because this solvent-free method perfectly fits in the current green chemistry trend. However, with classical thermal desorption systems, the use of activated charcoal as an adsorbent is not possible. Activated charcoal is a convenient, inexpensive, high trapping capacity material and compatible with a wide variety of organic, but the amount of provided energy during the heating desorption process is not sufficient to totally extract the most polar and low sterically hindered adsorbed compounds.
A new alternative microwave assisted thermal desorption technique allows the use of activated charcoal as sorbent. This technique provides almost instantly a high quantity of energy which enables a complete flash desorption of organics and does not require a secondary trap for cryofocalisation.
The goal of this study was to develop a re-usable sampling device usable in the microwave thermal desorption system directly after either active or passive sampling of organic compounds, without preparation steps.
The main challenge was to find an inert diffusion media in the diffusive ceramic tubes for passive samplings resistant to high temperatures without suffering irreversible structural damages. A carbonaceous material has been chosen as a diffusion material. The stability of its structure and diffusive properties as a function of the number of desorption cycles have been validated by microscopic analyses and also by samplings of aromatic compounds performed in controlled atmospheres. The results have shown the developed sampler appears to be suitable for passive samplings directly followed by assisted microwaves thermal desorption.
Microscopic observations made on the diffusion media after various desorption cycles have shown no structural modification or damage between a new diffusion material and up to a 50- cycle diffusion material. Thus, the developed sampler could at least be used 50 times with no variation in the results. Further investigations are made to evaluate the very limit of number of desorption cycles until a structural modification.
The uptake rates obtained from samplings in controlled atmospheres of toluene and m-xylene each at the threshold limit value TLV® (i.e. respectively 20 and 50 ppm) showed a low inter-sampler variability respectively of 1% and 2% for toluene and m-xylene. This experiment has been performed three times leading to similar results with only about 7% of variability for both compounds. The optimization of the quantification by external standard directly by the microwave thermal desorption system is still in progress.