## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Time (min)** & _m/z_ & **Compound** & **Confidence (\%)** & **Higher in** \\ \hline
33.0889 & 204 & Spiro[5.5]undeca-1,8-diene, 1,5,5,9-tetramethyl-, (R)- & 98 & COVID-19 \\ \hline
29.1006 & 224 & Cetene & 97 & COVID-19 \\ \hline
25.4963 & 138 & 2-Methoxy-5-methylphenol & 80 & Control \\ \hline
14.44 & 74 & Butanoic acid, 3-methyl- & 81 & COVID-19 \\ \hline
9.4973 & 91 & Toluene & 95 & COVID-19 \\ \hline
29.0055 & 159 & 1H-Inden-1-one, 2,3-dihydro-3,4,7-trimethyl- & 91 & COVID-19 \\ \hline
20.9414 & 170 & trans-Linalool oxide (furanoid) & 90 & Control \\ \hline
18.5347 & 68 & D-Limonene & 98 & COVID-19 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._and dimethyl sulfoxide (DMSO), or 2H-pyran-2-one, tetrahydro, as the most abundant compounds. 2,3-butanediol is a product of microbiota metabolism that contributes to reducing plasma cholesterol [49], and cyclopentanone has a protective effect against issues associated with diabetic nephropathy [50]. Other compounds, such as DMSO or 2H-pyran-2-one, tetrahydro, are products that do not form naturally and appear due to exposure [51]. With H2 SO4-treated urine samples, another series of volatile compounds were found (Table 4). Some are present in higher proportions in samples from COVID-19 patients, such as disulfide, dimethyl (DMDS); dimethyl trisulfide (DMTs); benzene, 1,2,4-trimethyl-; and 4-heptanone. In healthy individuals, we predominantly have 1, 1, 5-trimethyl-1, 2-dihydronaphthalene; trans-linalool oxide (furanoid); or phenol, 2-methoxy- (guaiacol). As noted, many of these compounds may have an external origin, such as diet [34] or exposure to certain environments. For example, 1,2-dihydronaphthalene is related to exposure to naphthalene, although it can also be a product of the degradation of lutein or beta-carotene [42]. The fact that many volatile compounds have a dietary origin makes interpreting them as biomarkers for SARS-CoV-2 infection very challenging. Additionally, it is important to consider that the bacterial microflora also plays a significant role in generating these volatile compounds present in urine, such as DMDS [52; 53; 54]. The microbiota also appears to be altered during COVID-19. The enormous complexity of human metabolism, along with the microbiota [54], has been highlighted in studies analyzing volatile compounds in the urine of cancer patients [55; 56]. As shown in Tables 2-4, the different pretreatments influence biomarker selection, favoring certain chemical families over others. In the analysis of untreated samples, hydrocarbons, ketones, organosulfur compounds, esters, and phenols are prominent. Without any additional treatment, this group presents a range of compounds that are inherently volatile and thermostable, making them suitable for GC-MS analysis. In the case of lyophilized samples, the primary compounds detected are alcohols, carboxylic acids, amines, lactams, and some halogenated compounds. The removal of water during lyophilization enhances the volatility of alcohols and carboxylic acids, improving their detectability in GC-MS [26]. Amines and nitrogen-containing compounds also benefit from lyophilization, as the process increases their concentration without altering their structure [26]. In contrast, samples treated with H2 SO4 show the presence of phenols, organosulfur compounds, monoterenes, ketones, and aromatic derivatives. The addition of sulfuric acid can protonate functional groups, particularly in phenols and amines, making some compounds more reactive [25]. This treatment can decompose unstable compounds, generating volatile derivatives or fragments that are detectable in GC-MS. Sulfuric acid may also induce dehydration in certain compounds, facilitating their volatilization [25].