## 3 Results and Discussion
### Suitability of Sorbent Materials
#### Hydrocarbons  
The results from the short-term stability studies for hydrocarbons are presented in Table 6 (recovery yield) and in Figures 1-3 (storage stability). \begin{table}
\begin{tabular}{c c c} \hline \hline  & **LOQ nmol/mol** & **Minimum Volume of Hydrogen to Sample (mL)** \\ \hline Methane & 500 & 6 \\ Ethane & 250 & 6 \\ Propane & 160 & 7 \\ Butanes & 125 & 7 \\ Acetone & 160 & 5 \\ Methanol & 500 & 3 \\ Ethanol & 250 & 4 \\ Heptane & 70 & 7 \\ Decane & 50 & 7 \\ Dichloromethane & 15 & 38 \\ Tetrachloroethylene & 15 & 20 \\ Tetrachlorohexafluorobutane & 15 & 11 \\ Dichlorobenzene & 15 & 22 \\ Carbonyl sulphide & 1.5 & 543 \\ Carbon disulfide & 1.5 & 429 \\ Tert-butyl mercaptan & 1.5 & 362 \\ Tetrahydrothiophene & 1.5 & 370 \\ Methyl-mercaptan & 1.5 & 679 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Calculation of the minimum volume of hydrogen that should be sampled on sorbent tubes to reach the thresholds set in ISO 14687-2. Figure 1: Storage stability of hydrocarbons on Chromosorb 106. (**a**) for butane, propane, isobutane, decane, ethanol; (**b**) for pentane, isopentane, acetone and heptane. Figure 2: Storage stability of hydrocarbons on TCC. (**a**) for butane, acetone, isobutane, decane, pentane; (**b**) for propane, isopentane, ethanol and heptane. \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 Propane & 91.3 & 2.1 & 95.0 & 1.0 & 9.9 & 49.9 \\ Isobutane & 96.5 & 0.7 & 88.3 & 1.9 & 20.4 & 15.0 \\ Butane & 97.5 & 0.4 & 96.3 & 1.4 & 61.3 & 24.6 \\ Ethanol & 102.1 & 7.2 & 145.9 & 36.6 & 81.0 & 1.3 \\ Isopentane & 95.9 & 0.4 & 85.4 & 0.8 & 54.0 & 8.6 \\ Acetone & 91.9 & 5.0 & 51.4 & 6.9 & 55.3 & 6.1 \\ Pentane & 95.1 & 0.7 & 93.1 & 1.4 & 87.9 & 3.3 \\ Heptane & 97.6 & 5.4 & 97.7 & 3.6 & 91.9 & 6.3 \\ Decane & 92.8 & 2.5 & 98.4 & 4.1 & 91.5 & 4.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Recovery yield obtained for hydrocarbons on 3 different sorbent tube. As it can be shown in Table 6, the relative standard deviations obtained from 4 to 6 replicates were found to be lower than 5% for most compounds or slightly above 5% for ethanol and heptane on Chromosorb 106, for acetone on TCC and for heptane, acetone and isopentane on Tenax TA. High relative standard deviation (>15%) were obtained for ethanol on TCC and for propane, isobutane and butane on Tenax TA. Unpredictable losses tend to indicate that the sorbent material is not suitable for the targeted compounds. Recovery yields above 90% were observed for most compounds on Chromosorb 106 and on TCC (through just under 90% for isobutane and isopentane on TCC) for most of the targeted compounds. However, on TCC, low recovery yields were observed for acetone (51.4%). For most of the compounds, the recovery yields on Tenax TA were under 80% with the exception of pentane (88%), heptane (92%) and decane (91.5%). As Tenax TA is a weak sorbent, compounds with low boiling points (<50 \({}^{\circ}\)C) are logically not well retained on this sorbent. However, with the low volume used in this study at the concentration tested, a measurable amount of isobutane (20%) and butane (60%) are retained on Tenax TA. When using two sorbents in series (for instance Tenax TA and Chromosorb 106), it would therefore be necessary to sum up the amount recovered on both sorbents. As it can be seen in Figures 1-3, most of the hydrocarbons evaluated in this study were determined to be very stable (D10 concentrations less than 10% below the initial concentrations) on Chromosorb 106 (except acetone), on TCC (except ethanol) and on Tenax TA (expect propane and acetone). Surprisingly, the concentration of acetone increased during the storage time (25-30% higher than the initial concentration) both on Chromosorb 106 and on Tenax TA.