# Methods for Sampling Biogas and Biomethane on Adsorbent Tubes after Collection in Gas Bags
## 4 Results and Discussions  
### Standard Deviation  
For one sample, ten tubes were taken using each of the tested methods. The precision of the results of the experiment is shown in Table 1 for eight different VOCs. The relative standard deviations of the measured VOC concentrations for most of the tests are lower than 5% rel., with the exception of methyl propyl disulphide using the EasyVOC method (6.2%) with undecane, \(\alpha\)-pinene and \(\beta\)-pinene (6.4%, 5.7% and 5.6%, respectively) using the sampling pump method (at a flow rate of 50 mL/min). The concentrations measured for 10 VOCs using the three methods (including two different flow rates for the sampling pump method) are shown in Figure 5a. The agreement between the methods is clearly satisfactory, with the exception of the sampling pump method at a flow rate of 50 mL/min for \(\alpha\)-pinene, \(\beta\)-pinene, 3-carene and p-cymene. These compounds have higher boiling points than the other compounds compared in this test. The chromatogram for method 1 is presented in Figure 5b. The current study focuses on real samples of biogas and biomethane. In a previous study [6] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rel. Standard Deviation \%** & **EasyVOC** & \multicolumn{2}{c}{**Syntinge**} & \multicolumn{2}{c}{**Sampling Pump**} \\ \hline Flow rate (mL/min) & 500 & 150 & 300 & 50 & 150 \\ Dimethyl sulphide & 1.3 & 1.1 & 1.6 & 3.8 & 1.9 \\ Dimethyl disulphide & 1.5 & 0.7 & 0.8 & 2.9 & 2.5 \\ Methyl propyl disulphide & 6.2 & 1.4 & 2.1 & 4.6 & 2.4 \\
2-methylfuran & 0.9 & 3.4 & 4.1 & 3.6 & 1.3 \\ Undecane & 3.4 & 0.3 & 2.8 & 6.4 & 3.1 \\ Toluene & 1.6 & 0.8 & 1.7 & 4.3 & 2.3 \\ \(\alpha\)-pinene & 4.1 & 2.0 & 1.5 & 5.7 & 2.6 \\ \(\beta\)-pinene & 5.0 & 1.8 & 1.7 & 5.6 & 2.6 \\
3-carene & 2.5 & 0.9 & 1.7 & 4.2 & 2.8 \\ p-cymene & 1.9 & 3.1 & 2.2 & 4.9 & 3.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relative standard deviation (\(n=10\)) of the 3 methods and VOCs. Figure 4: Variation in concentration for the 130 VOCs measured in this study. about the storage stability of impurities as siloxanes, sulphur compounds, halogenated and aromatic hydrocarbons, gravimetrically-prepared reference gas standards were stored in the same type of bags. The results were obtained using method 1 and show that satisfactory recovery yields (>90%) were obtained (storage duration < one day). In order to verify this preliminary observation, 14 different samples of biogas/biomethane (in total 130 VOCs) were analysed using the three different methods, using a flow rate of 200 mL/min for the syringe method, 500-600 mL/min for the EasyVOC method and 50 mL/min for the sampling pump method. The results were plotted in three different figures (Figures 6-8), using the EasyVOC method as the reference for both comparisons (Figure 6: comparison between syringe method and EasyVOC method; Figure 7: comparison between sampling pump method and EasyVOC method for samples with low water vapour content <0.8 vol-%; Figure 8: comparison between sampling pump method and EasyVOC method for samples with high water vapour content >0.8 vol-%). Figure 5: (a) Comparison of concentrations measured using the 3 different methods. (b) Chromatogram obtained with method 1. Figure 8: Difference in VOCs concentrations measured with the sampling pump method versus the EasyVOC method (water vapour \(>\)0.8 vol-%). Figure 6: Difference in VOCs concentrations measured with the syringe method versus the EasyVOC method. Figure 7: Difference in VOCs concentrations measured with the sampling pump method versus the EasyVOC method (water vapour \(<\)0.8 vol-%). The two methods give very comparable results (98 values out of 130 in the interval \(\pm\)5%, 22 values out of 130 in the intervals 5 to 10% or \(-\)5 to \(-\)10%, and only 10 values in the intervals 10 to 20 or \(-\)10 to \(-\)20%). The two methods give comparable results (Figure 8 (\(\pm\)10% with one outlier), if the samples contain a relatively high level of water vapour.