## 3 Results
### Characterization of Volatile Signature in Onions

Table 2: The volatomic signature of _Allium corpu_ L. from different geographic regions using headspace solid-phase microextraction (HS-SPME/GC-qMS). \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Peak n\({}^{*}\)**} & **RT (min) a** & **KI calcb ** & **KI LLt ** & **Chemical Groups/VOMs** & \multicolumn{2}{c}{**Similarity**} & \multicolumn{4}{c}{**GC Peak Area \(\times\)10\({}^{8}\) (USD)**} \\  & & & & & & (\%) & **Canigo** & **Santa Cruz** & **Ribeira Brava** & **Porto Moniz** \\ \hline  & \multicolumn{10}{c}{**Esters**} & & & & & & & & & \\
22 & 24.54 & 1264 & 1288 & Methyl heptanoate & 78 & 18.15 & (10) & - & - & - & - \\
38 & 38.02 & 1471 & 1446 & Propy hexaone & 72 & 14.98 & (18) & - & - & - & - \\
70 & 63.19 & 1900 & 1915 & 3-Methylbutyl benzoate & 74 & - & - & - & - & 74.31 & (8) \\
71 & 63.96 & 1915 & 1905 & Ethyl dibrochimanate & 80 & 5.01 & (18) & - & - & 38.21 & (18) \\
81 & 77.14 & 2049 & 2048 & Methyl cinnamate & 83 & 14.58 & (15) & - & - & - & - \\
**Allanes** & & & & & & & & & & \\
1 & 4.47 & 644 & - & 3-Methyl-1,4-pentadiene & 90 & - & 30.94 & (15) & 9.66 & (19) & 16.76 & (7) \\
4 & 5.41 & 786 & - & 2,4-Hexadiene isomer & 86 & - & 68.49 & (17) & 17.02 & (18) & 109.61 & (16) \\
58 & 52.27 & 1701 & 1719 & Undacional & 90 & - & - & - & 321.57 & (16) \\  & & & & & & & & & & \\
29 & 33.16 & 1392 & - & - & - & 619.12 & (14) & 1125.37 & (15) & - & 1044.64 & (17) \\
30 & 34.74 & 1447 & - & - & _m_/_z_ = 148,106,41,45 & - & 1367.84 & (1) & - & 14,622.71 & (9) & 8512.34 & (12) \\
43 & 39.75 & 1498 & - & -/z = 434.41, 457, 99,154 & - & 2302.17 & (15) & - & - & - & - \\
44 & 40.51 & 1510 & - & - & - (154.12124,437 & - & - & 844.13 & (18) & - & 455.38 & (2) \\
46 & 41.42 & 1526 & - & - & _m_/_z_ = 117, 146, 45 & - & - & 33.75 & (6) & - & 27.30 & (16) \\
49 & 44.36 & 1572 & - & _m_/_z_ = 87, 35, 451 & - & - & - & - & - & 49.14 & (8) \\
51 & 46.50 & 1655 & - & _m_/_z_ = 414, 573 & - & 19.75 & (10) & - & - & - & - \\
77 & 76.04 & 2043 & - & _m_/_z_ = 73,147,138 & - & - & 298.01 & (19) & - & 184.86 & (12) \\
79 & 76.58 & 2046 & - & _m_/_z_ = 73,147,138 & - & - & 325.23 & (16) & - & & \\
82 & 77.94 & 2053 & - & _m_/_z_ = 113,112,111,97 & - & - & - & 409.26 & (4) \\
86 & 84.34 & 2087 & - & _m_/_z_ = 134, 69 & - & - & 18.62 & (13) & - & 41.93 & (19) \\ \hline \hline \end{tabular}  
* Not detected; DDMF: 2-3-Dhydro-3,5-dihydroxy-6-methyl-4H-pyran4-one.  
\end{table} 
Table 2: _Cont._Table 2 shows all the detected VOMs, modes of identification, retention times, GC peak area (\(\times 10^{5}\)), and relative standard deviation (RSD) (\(n=3\)). The Kovats's index (KIs) for each VOM was calculated under identical chromatographic conditions as the samples and then compared with the scientific literature to ensure the unequivocal identification of the VOMs. A representative typical GC-qMS chromatogram of the volatile signature of Spanish onions cultivated at Canico, Santa Cruz, Ribeira Brava, and Porto Moniz is shown in Figure 1.  
Figure 1: Total ion chromatograms obtained by HS-SPME/GC-qMS analysis of onions (_Allium cpa_ L.) from Spanish variety from different geographical regions of Madeira Island. Attribution of the peak number is shown in Table 2.  
The results revealed that the volatile signature of Spanish onions from Porto Moniz was much richer (highest total GC peak area) compared to the volatile pattern of Spanish onions cultivated in the other investigated regions. A total of 58 VOMs were identified in Spanish onions cultivated in Canico, whereas in Spanish onions from Porto Moniz, Santa Cruz, and Ribeira Brava were identified 56, 40, and 28 VOMs, respectively. Among all detected VOMs, 17 were only found in Spanish onions cultivated in Canico, eight in Porto Moniz, two in Santa Cruz, two in Ribeira Brava, while 12 VOMs were common to all samples from four target regions (none of them belonging to sulfur compounds) (Table 2). It was well-known that sulfur compounds are the main volatiles in fresh onions.