## 3 Results and Discussion
### Suitability of Sorbent Materials
#### 3.2.2 Halogenated Hydrocarbons  
The results from the short-term stability studies for hydrocarbons are presented in Table 7 (recovery yield) and on Figures 4-6 (storage stability).  
As it can be seen in Table 7, chloromethane could only be recovered on TCC. Vinylchloride concentration showed variations greater than 13% on all sorbents (RSD 31% on Chromosorb 106, 16% on TCC and 13% on Tenax TA) as well as low recovery on Tenax TA (only 53%). Freon 113 was also poorly recovered on Tenax TA (40%). Surprisingly, chloroform which has a boiling point of 61 \({}^{\circ}\)C so in the boiling point interval where compounds are normally quantitatively retained, showed low  
Figure 3: Storage stability of hydrocarbons on Tenax TA. (**a**) for isobutane, butane, ethanol, pentane and isopentane; (**b**) for propane, acetone, decane and heptane.  
recovery on all sorbents (72% on Chromosorb 106, 68% on TCC and 67% on Tenax TA). All other compounds were recovered at more than 90% on all sorbents with the exception of trichloroethylene (87% on Chromosorb 106), dichloromethane and 1,2-dichlorethylene on Tenax TA (both 89%).  
As it can be seen on Figures 4 and 6, most of the halogenated hydrocarbons were found to be very stable (D14 concentrations less than 10% below the initial concentrations) on Chromosorb 106 and on Tenax TA (except chloromethane which could not be recovered in detectable concentration already on D0 on both sorbents). However, high variations of concentrations at D4, D5, D8 are observed for some compounds (freon 113, dichloromethane, tetrachlororethylene and 1,2-dichloropropane) on Chromosorb 106. Surprisingly, trichloroethylene concentration increased during the storage time on Chromosorb 106 (Figure 4c). As already observed on D0 (recovery yields), the behaviour of vinyl chloride on TCC was unpredictable (D0: 64 nmol/mol, D12: 37 nmol/mol, D14: 43 nmol/mol). Most of the halogenated hydrocarbons showed a slow decrease with time when stored on TCC (Figure 5) leading to a D14 concentration 15 to 25% lower than the initial concentrations with the exception of 1,2-dichlorobenzene and trichloroethylene which were both observed to be very stable. The decrease was even more pronounced for chloromethane (almost 30% lower concentration at D14 compared to D0) and with chloroform (slightly more than 30% lower concentration at D12 compared to D0) It is possible that once the tube was capped, part of the compounds may have been released from the weaker first sorbent bed and then preferentially migrated to the stronger beds, where they might have been (partially) irreversibly adsorbed resulting in incomplete recovery during analysis.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Chromosorb 106**} & \multicolumn{2}{c}{**TCC**} & \multicolumn{2}{c}{**Tenax TA**} \\ \cline{2-7}  & **Recovery** & **Rel. Standard** & **Recovery** & **Rel. Standard** & **Recovery** & **Rel. Standard** \\  & **(\%)** & **Deviation** & **(\%)** & **Deviation** & **(\%)** & **Deviation** \\ \hline Chloromethane & \textless{}5 & – & 57.1 & 9.0 & \textless{}5 & - \\ Vinyl chloride & 129.5 & 31.3 & 127.5 & 16.4 & 52.2 & 13.5 \\ Freon 113 & 106.3 & 7.1 & 102.1 & 2.8 & 39.6 & 7.8 \\ Dichloromethane & 99.4 & 0.2 & 95.4 & 0.6 & 88.6 & 3.6 \\
1,2-Dichloroethylene & 98.6 & 0.0 & 95.5 &