## 2 Materials and Methods
### Linear Regression Test, LOD, and LOQ of the GC-MS Method  
After reviewing the literature, twenty VOCs, which were recognized as targets, or as representatives of the major class of molecules, essential to the elaboration of the breath of the CKD-affected patients and/or healthy subjects were selected, and are reported in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \begin{tabular}{c} _Common_ \\ _Compound Name_ \\ \end{tabular} & _Molecular Class_ & \begin{tabular}{c} _Healthy_ \\ _Subjects_ \\ \end{tabular} & \begin{tabular}{c} _CKD_ \\ _Patients_ \\ \end{tabular} & \begin{tabular}{c} _CAS_ \\ _Number_ \\ \end{tabular} & \
\begin{tabular}{c} _M.W. (g-mol\({}^{-1}\))_ \\ \end{tabular} &
\begin{tabular}{c} _Bibliographic_ \\ _Ref._ \\ \end{tabular} \\ \hline
2-Butanone & Ketone & * & 78-93-3 & 72.11 & [47; 48; 49; 50] \\ \hline
3-Heptanone & Ketone & * & 106-35-4 & 114.19 & [47; 48; 49; 50] \\ \hline
1-Octyne & Alkyne & * & 629-05-0 & 110.20 & [47] \\ \hline Acetonitrile & Nitrogen & * & 75-05-8 & 41.05 & [40; 41; 42; 47] \\ \hline Benzaldehyde & Aldehyde, benzene & * & * & 100-52-7 & 106.12 & [44; 45; 46; 47; 48; 49; 50] \\ \hline Butanoic acid & Acid & * & 107-92-6 & 88.11 & [47; 48; 49; 50] \\ \hline Butanol & Alcohol & * & 71-36-3 & 74.12 & [47; 48; 49; 50] \\ \hline Decane & Alkane & * & * & 124-18-5 & 142.29 & [45; 47; 48; 49; 50] \\ \hline Dichloromethane & Chlorine & * & * & 75-09-2 & 84.93 & [47] \\ \hline Dimethyl sulfoxide & Sulfur compound & * & * & 67-68-5 & 78.13 & [47; 51] \\ \hline Dodecane & Alkane & * & * & 112-40-3 & 170.33 & [47; 48; 49; 50] \\ \hline Ethyl ether & Ether & * & 60-29-7 & 74.12 & [47] \\ \hline Ethylene diamine & Amine & * & 107-15-3 & 60.10 & [40; 41; 42; 47] \\ \hline \end{tabular}
\end{table}
Table 1: Selected VOCs.  
Stock solutions (1 mg\(\cdot\)mL\({}^{-1}\)) of each chosen volatile molecule (purity \(\geq\) 97%; _Sigma-Aldrich_, Milan, Italy) were prepared in methanol (purity \(\geq\) 98%; _Sigma-Aldrich_), except for hydrocarbons, which were solubilized in hexane (purity \(\geq\) 98%; _Sigma-Aldrich_), diluted to prepare working solutions, and stored at 8 \({}^{\circ}\)C.  
A working solution (1 \(\mu\)L), containing authentic standards (5, 10, 15, 25, 50, and 100 mg\(\cdot\)mL\({}^{-1}\)), was added into a biomonitoring sorbent tube and analyzed, following the procedure described above. The identification of the VOCs was performed with the MS database of the National Institute of Standards and Technology (NIST).  
The proposed GC-MS method was tested by linear regression analysis, plotting the peak area against the amount (ng) of each analyte in the biomonitoring sorbent tube. The LOD and LOQ were determined by LOD \(\cong\) (3\(\cdot\)sda)/b and LOQ \(\cong\) (10\(\cdot\)sda)/b, where sda is the standard deviation of the Y-intercept and b is the slope of the regression line. The reproducibility, designated as the intraday (\(n=3\)) and interdays (\(n=3\) over 7 days) percentages relative to the standard deviation (RSD %), was calculated at three concentration levels (five, ten, and twenty times the LOD and LOQ values in the TD tube) by analyzing daily prepared solutions with the same working mixtures stored at 8 \({}^{\circ}\)C.