## Materials and Methods
### Library Building  
After HS-SPME-GC-MS, chromatograms were generated (Figure 7, Table 2). Individual peaks were analysed by Automated Mass Spectral Deconvolution and Identification System (AMDIS, version 2.71, 2012, AMDIS is a freely available and sophisticated software for GC-MS data interpretation from NIST www.amdis.net) software, in conjunction with the National Institute of Standards and Technology mass spectral library (NIST, version 17, 2019, Purchased from SS Scientific Limited, Eastbourne, UK). Peaks were added to a local library: if they had a forward match of greater than 740/1000, they were silica based degradation products (5 compounds), if they had a good shape total ion count trace (2 unknown compounds), or if they had previously been identified in other studies in our lab. The final local library contained 173 unique VOCs (Table S1).  
A batch report was generated from AMDIS using the local library with deconvolution settings as follows: component width of 10, adjacent peak subtraction of one, low resolution, low sensitivity, and low shape requirements. Freely available R package Metab (Bioconductor) was used to generate a spreadsheet of VOCs per sample, using a half a minute time-window [55]. Contaminants identified with  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Label**} & **Retention Time** & **Compound Name (IUPAC Name)** \\  & **(min)** & **Compound Name (IUPAC Name)** \\ \hline
1 & 16.93 & 2,2,4,4,6,6-hexamethyl-1,3,5,2,4,6-trioxatrisilinae (Contaminant) * \\
2 & 20.88 & heptan-4-one * \\
3 & 21.44 & heptan-3-one * \\
4 & 24.67 & 2-methyl-5-methylsulfanylfuran \\
5 & 26.01 & 1-methyl-4-propan-2-ylbenzene (\(p\)-Cimene) \\
6 & 26.93 & 2-ethylhexan-1-ol * \\
7 & 27.76 & Phenol \\
8 & 28.55 & 1-methyl-4-prop-1-en-2-ylbenzene-or-1-methyl-2-prop-1-en-2-ylbenzene \\
9 & 30.61 & 4-methylphenol \\
10 & 31.83 & (1R,2R,5R)-5-methyl-2-propan-2-ylcyclohexan-1-ol \\
11 & 32.14 & 5-methyl-2-propan-2-ylcyclohexan-1-ol(Menthol) \\
12 & 35.42 & 3-methyl-6-propan-2-ylcyclohex-2-en-1-one * \\
13 & 37.32 & 2-beta-1,3-dienyl-1,3,5-trimethylbenzene \\
14 & 16.93 & 2,2,4,4,6,6-hexamethyl-1,3,5,2,4,6-trioxatrisilinae (Contaminant) * \\
15 & 20.88 & heptan-4-one * \\
16 & 21.44 & heptan-3-one * \\
17 & 21.71 & heptan-2-one \\
18 & 22.61 & (E)-2-methylhept-2-enal \\
19 & 23.10 & 2,2,4,4,6,6,8,8-octamethyl-1,3,5,7,2,4,6,8-tetraoxactrasilocane (Contaminant) * \\
20 & 23.91 & Oxime-, methoxy-phenyl-(Contaminant) \\
21 & 26.93 & 2-ethylhexan-1-ol * \\
22 & 28.53 & Tetramethylsilane (TMS) (Contaminant) \\
23 & 35.42 & 3-methyl-6-propan-2-ylcyclohex-2-en-1-one * \\ \hline \hline \end{tabular}  
* Some compounds appear in this table twice as they are highly abundant in both treatment conditions.  
\end{table}
Table 2: Most abundant chemicals in chromatogram.  
Figure 7: Example of typical chromatograms of duplicate 1 mL urine samples generated from HS-SPME-GC-MS following treatment with 0.2 mL of 5 M H\({}_{2}\)SO\({}_{4}\) (red) or 5 M NaOH (blue). The 13 most abundant chemicals in urine treated with 5 M H\({}_{2}\)SO\({}_{4}\) and nine most abundant chemicals in urine treated with 5 M NaOH are labelled and further information regarding these peaks can be seen in Table 2.  
system suitability tests were removed before statistical analysis as described above. Statistics and figures were produced using Excel, the freely-available MetaboAnalyst website (McGill University, Canada, Xia Lab, [[https://www.metaboanalyst.ca/](https://www.metaboanalyst.ca/)]([https://www.metaboanalyst.ca/](https://www.metaboanalyst.ca/))), or free software environment for statistical computing and graphics R software [56; 57].  
Data was normalized by median, transformed logarithmically, and auto scaled accordingly to ensure that all metabolites were given equal weight during the analysis and to increase the interpretability of the models produced.