## 2 Results and Discussion
### Analytical Performance  
The demonstration that the method is of high quality is a crucial step in method development [34]. The validation of the method for non-targeted metabolomics comprises in most cases solely the analysis of precision [2]. Therefore, to check the reliability of the methodology, an external standard method was applied. Seven commercial standards, representing acids and aldehydes with different chain lengths, were selected, and the standard mixture was used for precision, sensitivity and linearity evaluation. The results are presented in Table 2.  
The intraday and interday precision ranged from 6.1% to 14.9%, and from 9.1% to 26.4%, respectively. [PERSON] et al. [34] recommended that the Relative Standard Deviation (RSD) values should not exceed 30% in metabolomics studies, therefore the results obtained in our study can be considered satisfactory, especially for the manual injection applied in our study. The correlation coefficients (R\({}^{2}\)) of the calibration curves for all selected standards ranged between 0.98 and 0.99, indicating that the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Retention** & **Compound** & **Intraday Precision** & **Interday Precision** & **Linear Range** & **LOD ”** & **LOQ ”** \\
**Time** & **Compound** & **[RSD’\%]** & **[RSD’\%]** & **[JimQ/I]** & **R\({}^{2}\)** & **[imQ/I]** & **[imQ/I]** \\ \hline
9.78 & heptanal & 9 & 9 & 8.857–0.883 & 0.984 & 4.4 & 14.8 \\
13.11 & octanal & 9 & 19 & 8.004–6.035 & 0.986 & 4.0 & 13.3 \\
16.15 & nonanal & 13 & 17 & 7.268–58.142 & 0.976 & 3.6 & 12.1 \\
19.84 & decanal & 6 & 16 & 6.642–53.137 & 0.983 & 3.3 & 11.1 \\
29.70 & hexanoic acid & 15 & 15 & 9.997–99.797 & 0.985 & 10.0 & 33.3 \\
32.51 & heptanoic acid & 13 & 26 & 8.814–70.515 & 0.988 & 8.8 & 29.4 \\
35.05 & octanoic acid & 11 & 15 & 3.944–63.102 & 0.990 & 3.9 & 13.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Validation parameters calculated for a mixture of commercial standards. Compounds are ordered with respect to their increasing retention times.  
Figure 1: The comparison of total ion mass chromatograms of volatile compounds (VCs) extracted from urine sample using solid-phase microextraction (green chromatogram) and the optimized method (red chromatogram). The tentatively identified compounds: (**1**) Allyl isothiamate; (**2**) Tetradecane; (**3**) Acetic acid; (**4**) 2-Butyl-1-octanol; (**5**) Diethyl sulfoxide; (**6**) hexadecane; (**7**) tetrahydro-6-methyl-2\(H\)-Pyran-2-one; (**8**) 2-Methoxy-phenol; (**9**) Dimethyl sulfone; (**10**) Heptanoic acid; (**11**) 2-Methyl-octanoic acid; (**12**) \(p\)-Cresol; (**13**) Erucin; (**14**) Nonanoic acid; (**15**) Octenoic acid; (**16**) 2-Methoxy-4-vinylphenol; (**17**) \(n\)-Decanoic acid; (**18**) Divinyl sulphide; (**19**) 1-Hexadecanol; (**20**) Benzoic acid; (**21**) 7-Methylindole (**22**) Benzeneacetic acid; (**23**) Apocynin; (**24**) Benzamide; (**25**) \(n\)-Hexadecanoic acid; (**26**) Octadecanoic acid; (**27**) Caffeine; (**28**) 4-heptanone; (**29**) \(p\)-Cymene; (**30**) 4-Ethenyl-1,2-dimethylbenzene.  
method has a highly linear response for the concentration ranges presented in Table 2. The limit of detection (LOD) and limit of quantification (LOQ) values were found to be less than 10 and less than 33.3 umol/L, respectively, for all the analyzed standards. However, the sensitivity of the method was not the priority of this study.