## 2 Methods
### Selection of Sorbents  
The sorbent selected must be sufficiently strong to retain the target analytes during sampling, but weak enough to release them efficiently during the thermal desorption phase. Low-volatility analytes adsorb onto the weakest sorbent and high-volatility analytes adsorb onto the strongest sorbent. Methane and ethane can only be trapped at very low temperatures (\(<\)\(-\)20 \({}^{\circ}\)C) on very strong sorbents; for example, carbon molecular sieves such as Carbosieve SIII. This cannot be done easily in the field [12; 13]. Using sorbent with high strength may result in the irreversible adsorption of higher boiling point species. However, a weaker sorbent like Carboxen 569, and even lower temperatures (down to \(-\)80 \({}^{\circ}\)C), has been shown to trap both low- and medium-boiling-point hydrocarbons (nC\({}_{1}\)-nC\({}_{4}\)) [14]. Very volatile organic compounds between C\({}_{3}\) to C\({}_{6}\) of different volatility have been successfully detected using Carbograph 5 TD (20/40 mesh) [15]. Carboxen 1016 has also been proven to be efficient since it is similar to Carboxen 569, but is not releasing sulphur dioxide (SO\({}_{2}\)) during heating, which makes it a good candidate to trap low boiling point compounds. For the higher boiling point hydrocarbons such as decane and octane, Tenax TA is the most commonly used sorbent [16]. For the tests performed in this study regarding linear alkanes with 1 to 4 hydrocarbons, acetone, methanol and ethanol the following sorbents were selected; Tenax TA (Perkin Elmer, U.K.), 200 mg; a multibed sorbent consisting of Tenax TA (weak), Carboxen 1003 (medium), Carbograph 1 (strong) (Markes International Ltd., Lantrisant, UK) totally 200 mg, 1/3 of each sorbent material and two sorbents in series (medium/weak): Chromosorb 106 (Markes International Ltd., Lantrisant, UK + Tenax TA (Perkin Elmer, Lantrisant, UK) each containing 200 mg. The trapping of the halogenated compounds selected in this study would require a medium sorbent (for chloroform and dichloromethane) and a weak sorbent (for tetrachloroethylene and dichlorobenzene). However, studies [17; 18] show that even strong sorbents and weak sorbents can efficiently trap chloroform and dichloromethane. For weak sorbents, small sampling volumes must be selected in order to avoid the loss of the more volatile compounds by breakthrough. If only strong sorbents are used, the risk that tetrachloroethylene and dichlorobenzene will be irreversibility adsorbed is high. In order to avoid this risk altogether, it is necessary to trap these compounds on a weaker sorbent first. Therefore, the same adsorbent materials than those selected for hydrocarbons were chosen for halogenated compounds. Tenax TA is probably the most popular sorbent used in thermal desorption. It presents the advantages to be a low bleeding material, exhibits a low level of impurities and is hydrophobic. Because methanol is only slightly retained, spiking Tenax TA with a liquid standard in methanol is a reliable option for calibration. A low volume should be sampled to avoid breakthrough of low volatile compounds. The multibed sorbent is expected to provide the optimal combination for trapping a large range of volatilities. Tenax TA was shown in the literature study to efficiently trap dichlorobenzenes and tetrachloroethylene while Carboxen has been shown to trap chloroform and dichloromethane efficiently. Finally, the train of two sorbents (medium/weak) in series allows in some aspects; more flexibility. Larger volumes can be sampled (as there is more sorbent in one tube than in a multi-bed tube, the breakthrough volume being dependent upon the weight of the sorbent) and if necessary, different desorption temperatures, desorption flows and desorption times can be used for each tube. COS and CS\({}_{2}\) are notoriously difficult to trap and require the trapping to be performed at low temperatures (at least \(-\)5 \({}^{\circ}\)C) on very strong sorbents for example carbon molecular sieves, such as Carbosieve SIII [19; 20; 21; 22].