## 2 Materials and Methods
### GC-qMS Conditions  
An Agilent 6890N (Palo Alto, CA, USA) gas chromatography system combined with an Agilent 5975 quadrupole mass selective detector and splitless injector were used. Chromatographic separation was carried out by a BP-20 (30 m \(\times\) 0.25 mm i.d. \(\times\) 0.25 \(\upmu\)m film thickness) fused silica capillary column provided by SGE (Darmstadt, Germany) with helium (Helium N60, Air Liquid, Portugal) as carrier gas at a flow rate of 1 mL/min (column-head pressure: 13 psi). An insert of 0.75 mm i.d. was used and the injector temperature was fixed at 250 \({}^{\circ}\)C. The GC oven temperature was held for 5 min at 45 \({}^{\circ}\)C, ramped to 150 \({}^{\circ}\)C at 2 \({}^{\circ}\)C/min (held for 10 min) and increased from 150 to 220 \({}^{\circ}\)C (held for 10 min) at 2 \({}^{\circ}\)C/min. The manifold, GC-qMS interface, and quadrupole temperatures were held at 180, 220, and 180 \({}^{\circ}\)C, respectively. In addition, MS detection was set in full scan, the ion energy used for the electron impact (EI) was 70 eV and the source temperature was 180 \({}^{\circ}\)C. The electron multiplier was set to the auto-tune procedure. The mass acquisition range, made in full scan mode, was 30-300 \(m\)/z, 1.9 spectra/s. Furthermore, VOMs identification was achieved in the following ways:  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Location** & **Latitude \({}^{\textbf{a}}\)** & **Longitude \({}^{\textbf{a}}\)** & **Altitude (m) \({}^{\textbf{a}}\)** & **Mean** & **Mean** & **Köppen-Geiger** \\  & & & & **Temperature** & **Rainfall** & **Climate** \\  & & & & **(\({}^{\textbf{C}}\)) \({}^{\textbf{b}}\)** & **(mm) \({}^{\textbf{b}}\)** & **Classification \({}^{\textbf{b}}\)** \\ \hline Porto Moniz & 32\({}^{\circ}\)52\({}^{\prime}\)0.06\({}^{\prime\prime}\) N & 17\({}^{\circ}\)10\({}^{\prime}\)14.19\({}^{\prime\prime}\) W & 40 & 18.9 & 559 & Csa \\ Ribeira Brava & 32\({}^{\circ}\)40\({}^{\prime}\)26.36\({}^{\prime\prime}\) N & 17\({}^{\circ}\)3\({}^{\prime}\)3.57\({}^{\prime\prime}\) W & 360 & 17.6 & 614 & Csb \\ Canigo & 32\({}^{\circ}\)39\({}^{\prime}\)6.14\({}^{\prime\prime}\) N & 16\({}^{\circ}\)50\({}^{\prime}\)7.93\({}^{\prime\prime}\) W & 163 & 17.6 & 614 & Csb \\ Santa Cruz & 32\({}^{\circ}\)41\({}^{\prime}\)21.41\({}^{\prime\prime}\) N & 16\({}^{\circ}\)47\({}^{\prime}\)26.66\({}^{\prime\prime}\) W & 31 & 18.8 & 592 & Csa \\ \hline \hline \end{tabular}
\begin{tabular}{c} Csa: hot-summer Mediterranean climate. Csb: warm-summer Mediterranean climate; \({}^{\textbf{a}}\) Information obtained from www.google.com/maps; \({}^{\textbf{b}}\) Information obtained from [[https://es.climate-data.org/](https://es.climate-data.org/)]([https://es.climate-data.org/](https://es.climate-data.org/)). \\ \end{tabular}
\end{table}
Table 1: Geographic coordinates and climate conditions of the four regions of Madeira Island where the onions were cultivated.  
1. Comparison the GC retention times (RT) and mass spectra with those of the standard, when available;
2. All mass spectra were also compared with the data system library (NIST, 2005 software, Mass Spectral Search Program v.2.0d; Nist 2005, Washington, DC, USA). Single VOM peak was considered as identified compound when its experimental spectrum corresponded with a score of over 80% that present in the library;
3. Kovats's retention index (KI) values were determined according to the van [PERSON] Dool and Kratz equation [17]. For the determination of the KI, a C\({}_{8}\)-C\({}_{20}\)\(n\)-alkanes series were used and the values were compared, when available, with values reported in the scientific literature for similar columns [18; 19; 20].