## 4 Discussion  
Initially, it was demonstrated that the optimized TD-GC-MS operating conditions were able to adequately detect each selected standard molecule without any overlap between them. For all considered analytes, good linearity was ensured in the quantitative range explored, with the resulting R\({}^{2}\) values always greater than 0.9719, and the linearity range being correct for the significant determination of the considered compounds. Moreover, the estimated LOD and LOQ values were in line with those previously reported in the literature. For example, [PERSON] et al. reported for alkane (e.g., pentane and hexane), a LOD of about a few dozen pg.mL\({}^{-1}\) alongside LOQ values of a maximum of 200 pg.mL\({}^{-1}\)[51]. Similar values were recorded for nitrogen, sulfur-containing compounds, and ketones [51,52]. The breath analysis results showed that the two populations considered (healthy people and CKD-affected patients) were characterized by the presence of the same substances, except for the nitrogen-containing compounds (acetonitrile, ethylenediamine, propylamine), which were present only in the exhaled breath of the CKD-affected patients. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline _Peak n\({}^{\circ}\)_ & _RT a (min)_ & \begin{tabular}{c} _Common Compound_ \\ _Name_ \\ \end{tabular} & \begin{tabular}{c} _Match_ \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} _Probability_ \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} _Standard identity_ \\ _Confirmation b\({}^{\text{b}}\)_ \\ \end{tabular} & \
\begin{tabular}{c} _Healthy_ \\ _Subjects_ \\ \end{tabular} &
\begin{tabular}{c} _CKD_ \\ _Patients_ \\ \end{tabular} \\ \hline
70 & 29.6 & \
\begin{tabular}{c} Silane, \\ ethyl-dimethyl-phenyl \\ \end{tabular} & 813 & 62 &
\begin{tabular}{c} yes \\ \end{tabular} & yes & yes \\ \hline
71 & 29.8 & 4-Phenyl benzofurane & 822 & 56 &
\begin{tabular}{c} yes \\ \end{tabular} & yes & yes \\ \hline
72 & 30.5 & Tri-tetra-contane & 812 & 56 &
\begin{tabular}{c} yes \\ \end{tabular} & yes & yes \\ \hline
73 & 30.7 & Hexestrol & 828 & 52 &
\begin{tabular}{c} yes \\ \end{tabular} \\ \hline
74 & 31.5 & Unidentified & & yes & yes \\ \hline \hline \end{tabular}  
* Values expressed as mean (s.d.). b A Authenticated using the NIST library and standard injection.  
\end{table} 
Table 6: _Cont._ \begin{table}
\begin{tabular}{c c c} \hline \hline _Common Compound Name_ & \multicolumn{2}{c}{_pg-mL\({}^{-1}\) in Exhaled Breath_} \\ \cline{2-3}  & \multicolumn{2}{c}{_Healthy Subjects_} & _CKD Affected Patients_ \\ \hline
2-Butanone & 20–70 & 10–30 \\ \hline
3-Heptanone & LOD-12 & 10–40 \\ \hline Acetonitrile & n.d. & 7–20 \\ \hline Benzaldehyde & n.d.-50 & n.d.-LOD \\ \hline Butanoic acid & LOD-60 & LOD-15 \\ \hline Decane & n.d.-50 & 25–40 \\ \hline Dichloromethane & LOD-20 & LOD-15 \\ \hline dodecane & n.d.-60 & 40–70 \\ \hline Ethylenediamine & n.d. & n.d.-LOD \\ \hline Hexanal & n.d.-50 & 35–150 \\ \hline Hexanoic acid & n.d.-60 & 90–120 \\ \hline Propylamine & n.d. & 13–30 \\ \hline Toluene & n.d.-20 & 5–20 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The concentration range of the thirteen selected VOCs identified in the exhaled breath samples for both healthy subjects and/or CKD-affected patient populations. As reported in the literature, nitrogen-based substances are an indication of renal failure. In fact, as previously underlined, ammonia and amines have been shown to be elevated in the breath of subjects affected by CKD [40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52]. In general, higher levels of aldehyde compounds are expected in the breath of CKD patients. These compounds can originate from membrane phospholipids during peroxidation processes by reactive oxygen species. Oxidative stress has been related to chronic renal failure [44]. Therefore, aldehydes can be considered biomarkers of oxidative stress [44]. For instance, [PERSON] et al.