Table 2.
The volatomic signature of Allium cepa L. from different geographic regions using headspace solid-phase microextraction (HS-SPME/GC-qMS).
| Peak nº | RT (min) a | KI calc b | KI Lit c | Chemical Groups/VOMs | Similarity (%) | GC Peak Area × 105 (RSD) | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Caniço | Santa Cruz | Ribeira Brava | Porto Moniz | ||||||||||
| Sulfur Compounds | |||||||||||||
| 2 | 4.56 | 649 | 635 | Methanethiol | 90 | - | - | 14.66 | (19) | - | |||
| 3 | 4.82 | 661 | 660 | Carbon disulfide | 83 | 129.23 | (17) | 59.71 | (15) | - | 107.84 | (9) | |
| 5 | 5.82 | 802 | 770 | 1-Propanethiol | 95 | 34.91 | (17) | 20.56 | (12) | 69.28 | (17) | 155.06 | (17) |
| 7 | 7.73 | 954 | 961 | Thiophene | 93 | 196.25 | (4) | 613.09 | (17) | 467.48 | (5) | 1154.56 | (10) |
| 8 | 12.93 | 1068 | 1068 | Dimethyl disulfide | 94 | 18.93 | (13) | - | - | - | |||
| 11 | 16.40 | 1134 | 1127 | 3-Methyl-thiophene | 86 | - | 4.41 | (7) | - | 4.70 | (17) | ||
| 12 | 16.95 | 1143 | 1150 | Thiophane | 86 | - | 13.03 | (17) | - | - | |||
| 16 | 18.75 | 1173 | 1191 | 1-(Methylthio)ethanethiol | 94 | 58.51 | (16) | 70.04 | (19) | 92.79 | (9) | 109.63 | (11) |
| 17 | 19.85 | 1190 | 1215 | Methyl 2-propenyl disulfide | 64 | - | 12.07 | (14) | - | 8.01 | (14) | ||
| 19 | 21.24 | 1213 | 1221 | Methoxymethylbutanethiol | 91 | 25.45 | (16) | - | 137.96 | (17) | 152.55 | (19) | |
| 20 | 22.63 | 1235 | 1224 | Methyl propyl disulfide | 87 | 203.82 | (12) | 1280.73 | (14) | 1482.78 | (9) | 1844.03 | (5) |
| 21 | 23.20 | 1244 | 1228 | Methyl isothiocyanate | 82 | 201.68 | (10) | 83.93 | (9) | 380.48 | (19) | 48.61 | (17) |
| 23 | 24.64 | 1265 | 1253 | 2,4-Dimethyl-thiophene | 74 | 228.34 | (16) | 13.99 | (14) | - | 31.06 | (5) | |
| 24 | 25.10 | 1272 | 1273 | Hexanethiol | 64 | - | 222.73 | (10) | - | 115.76 | (15) | ||
| 26 | 30.84 | 1358 | 1370 | Dipropyl disulfide | 91 | 1528.80 | (18) | 2364.77 | (13) | 14,144.44 | (7) | 11201.60 | (15) |
| 27 | 31.10 | 1362 | 1364 | Dimethyl trisulfide | 97 | 812.81 | (13) | - | 338.43 | (16) | - | ||
| 31 | 35.09 | 1423 | 1428 | 1-Propenyl propyl disulfide isomer | 73 | 4.35 | (11) | - | - | - | |||
| 32 | 36.05 | 1439 | 1429 | Thenylthiol | 91 | 29.44 | (12) | - | 75.95 | (18) | - | ||
| 33 | 36.31 | 1444 | 1438 | 2-Pentylthiophene | 74 | - | 27.18 | (5) | 215.22 | (1) | 34.47 | (18) | |
| 36 | 36.81 | 1452 | 1458 | Methional | 79 | - | - | - | 291.57 | (11) | |||
| 39 | 38.28 | 1475 | 1476 | 2-Methyl-1,3-dithiane | 78 | - | - | - | 480.56 | (10) | |||
| 40 | 38.71 | 1482 | 1506 | Methyldihydrothiophenone | 87 | - | - | - | 14.10 | (9) | |||
| 42 | 39.35 | 1492 | 1516 | Methyl propyl trisulfide | 78 | 35.29 | (7) | 45.86 | (17) | - | 313.73 | (10) | |
| 47 | 41.98 | 1535 | 1520 | Methyldithiofurane | 78 | - | 95.76 | (16) | - | 155.88 | (15) | ||
| 52 | 46.52 | 1606 | 1606 | Sulfurol | 78 | 4.14 | (13) | - | - | - | |||
| 53 | 48.90 | 1646 | 1622 | Phenylethylthiol | 95 | 3110.10 | (17) | 768.23 | (10) | 3310.32 | (8) | 2998.56 | (19) |
| 57 | 51.35 | 1686 | 1667 | Methyl-(methyldithio)furan | 80 | 74.19 | (11) | - | - | 302.68 | (5) | ||
| 59 | 52.50 | 1704 | 1700 | 3-(Acetylthio)-2-methylfuran | 82 | 2629.08 | (16) | - | 354.53 | (9) | 44.83 | (9) | |
| 60 | 53.19 | 1716 | 1723 | Methionol | 89 | - | - | - | 946.48 | (19) | |||
| 62 | 54.30 | 1734 | 1738 | Dipropyl trisulfide | 82 | - | 28.46 | (16) | - | 215.70 | (13) | ||
| 63 | 54.46 | 1737 | - | 3,5-Diethyl-1,2,4-trithiolane isomer | 78 | - | 521.55 | (10) | - | - | |||
| 64 | 54.71 | 1741 | 1759 | 3-Methylthiopropyl cyanide | 78 | 133.36 | (18) | - | - | 902.24 | (9) | ||
| 67 | 58.41 | 1798 | 1785 | Acetylthiophene | 75 | 33.52 | (10) | 191.17 | (15) | 173.31 | (12) | 295.88 | (18) |
| 69 | 62.96 | 1896 | 1896 | 5-(Methylthio)-4-pentenenitrile | 72 | 24.88 | (12) | 87.04 | (5) | - | - | ||
| 75 | 70.43 | 2010 | 1998 | Methyl methanethiosulfonate | 75 | 19.28 | (14) | 65.46 | (13) | 44.65 | (10) | - | |
| 76 | 73.52 | 2029 | 2038 | 5-Methoxy-thiazole | 72 | 40.10 | (13) | 314.49 | (16) | 124.92 | (16) | 82.21 | (11) |
| Aldehyde | |||||||||||||
| 6 | 5.93 | 806 | 832 | Butanal | 72 | 105.00 | (14) | 93.41 | (6) | 177.39 | (19) | 115.90 | (9) |
| 9 | 13.36 | 1077 | 1084 | Hexanal | 72 | 122.25 | (13) | 6.62 | (17) | 119.68 | (4) | 22.95 | (17) |
| 10 | 14.02 | 1089 | 1101 | 2-Methyl-2-butenal isomer | 84 | 28.82 | (19) | - | - | - | |||
| 13 | 17.28 | 1149 | 1141 | 2-Methyl-4-pentenal isomer | 93 | 8.74 | (14) | 33.48 | (15) | - | 55.06 | (11) | |
| 14 | 17.42 | 1152 | 1155 | 2-Methyl-2-pentenal isomer | 94 | 696.86 | (11) | 1284.04 | (19) | 1448.52 | (16) | 1618.87 | (18) |
| 25 | 27.57 | 1307 | 1280 | Octanal | 74 | 26.81 | (17) | 10.77 | (18) | 28.03 | (15) | - | |
| 41 | 39.12 | 1487 | 1484 | Decanal | 96 | - | 73.93 | (19) | - | 13.08 | (15) | ||
| 45 | 41.16 | 1521 | 1527 | 2-Nonenal isomer | 94 | 16.10 | (6) | 39.40 | (15) | - | - | ||
| 56 | 50.92 | 1680 | 1701 | 2,4-Nonadienal isomer | 75 | 10.28 | (2) | - | - | - | |||
| Furanic Compounds | |||||||||||||
| 18 | 20.80 | 1205 | 1240 | 2-Pentylfuran | 91 | 9.54 | (14) | - | 77.41 | (5) | - | ||
| 37 | 37.25 | 1459 | 1458 | Furfural | 91 | 83.29 | (8) | - | - | - | |||
| 48 | 43.73 | 1563 | 1560 | 5-Methyl-2-furfural | 91 | 46.61 | (16) | - | - | - | |||
| 50 | 44.59 | 1576 | 1580 | Dimethylmethoxyfuranone | 74 | 877.44 | (15) | - | - | 23,151.04 | (19) | ||
| 54 | 49.03 | 1649 | 1661 | 2-Furanmethanol | 95 | 136.05 | (14) | - | - | - | |||
| 61 | 54.09 | 1731 | 1710 | 5-Ethylfuran-2-(5H)-one | 80 | 12.79 | (18) | - | - | - | |||
| 65 | 56.53 | 1769 | 1745 | Ethyl furanone | 72 | 229.33 | (15) | - | - | 12,652.82 | (17) | ||
| 74 | 69.12 | 2002 | 2005 | Acetyloxy-dimethylfuranone | 72 | 5323.38 | (10) | - | 4723.21 | (5) | 6125.12 | (2) | |
| 78 | 76.51 | 2045 | 2043 | Furaneol | 79 | 5.46 | (19) | - | - | - | |||
| 85 | 84.15 | 2086 | 2087 | 5-Hydroxymethyl-2-furfural | 83 | 167.64 | (16) | 12.16 | (19) | - | - | ||
| 87 | 87.35 | 2102 | 2111 | Methyl furaneol | 86 | - | - | 85.87 | (7) | - | |||
| Ketones | |||||||||||||
| 28 | 32.12 | 1377 | 1388 | 2-Nonanone | 86 | 684.65 | (10) | 22.86 | (14) | - | 28.48 | (9) | |
| 55 | 50.57 | 1674 | 1645 | Acetophenone | 80 | - | - | - | 75.64 | (14) | |||
| 83 | 78.28 | 2055 | - | DDMP | 91 | 193.67 | (12) | - | - | 39.42 | (17) | ||
| Lactones | |||||||||||||
| 66 | 57.27 | 1781 | 1770 | γ-Heptalactone | 98 | 84.42 | (10) | - | - | 71.65 | (8) | ||
| 72 | 67.41 | 1977 | 1977 | Whiskey lactone isomer | 78 | 47.51 | (7) | - | - | 6038.33 | (12) | ||
| 73 | 68.45 | 1995 | 1985 | Whiskey lactone isomer | 86 | 105.85 | (13) | - | - | - | |||
| 80 | 77.02 | 2048 | 2042 | γ-Nonalactone | 86 | 198.88 | (9) | - | 52.20 | (18) | 1470.16 | (4) | |
| Terpenes | |||||||||||||
| 15 | 18.43 | 1168 | 1178 | Limonene isomer | 94 | 4.96 | (6) | - | 222.04 | (11) | 64.73 | (9) | |
| 68 | 59.44 | 1820 | 1840 | Geranyl acetone | 86 | - | - | - | 14.95 | (15) | |||
| Acids | |||||||||||||
| 34 | 36.59 | 1448 | 1449 | Acetic acid | 88 | 339.21 | (16) | 18.77 | (17) | - | 125.42 | (17) | |
| 84 | 82.21 | 2076 | 2083 | Octanoic acid | 94 | 18.42 | (10) | - | - | - | |||
| Esters | |||||||||||||
| 22 | 24.54 | 1264 | 1288 | Methyl heptanoate | 78 | 18.15 | (10) | - | - | - | |||
| 38 | 38.02 | 1471 | 1446 | Propyl hexanoate | 72 | 14.98 | (18) | - | - | - | |||
| 70 | 63.19 | 1900 | 1915 | 3-Methylbutyl benzoate | 74 | - | - | - | 74.31 | (8) | |||
| 71 | 63.96 | 1915 | 1906 | Ethyl dihydrocinnamate | 80 | 5.01 | (18) | - | - | 38.21 | (18) | ||
| 81 | 77.14 | 2049 | 2048 | Methyl cinnamate | 83 | 14.58 | (15) | - | - | - | |||
| Alkanes | |||||||||||||
| 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) | |
| Alcohols | |||||||||||||
| 58 | 52.27 | 1701 | 1719 | Undecanol | 90 | - | - | - | 321.57 | (16) | |||
| Unknown | |||||||||||||
| 29 | 33.16 | 1392 | - | m/z = 148,106,41,45 | - | 619.12 | (14) | 1125.37 | (15) | - | 1044.64 | (17) | |
| 30 | 34.74 | 1417 | - | m/z = 148,106,41,45 | - | 1367.84 | (1) | - | 14,622.71 | (9) | 8517.34 | (12) | |
| 43 | 39.75 | 1498 | - | m/z = 43,41, 45, 47, 99, 154 | - | 2302.17 | (15) | - | - | - | |||
| 44 | 40.51 | 1510 | - | m/z = 154,112,43,47 | - | - | 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, 73, 45, 41 | - | - | - | - | 49.14 | (8) | |||
| 51 | 46.50 | 1605 | - | m/z = 41,45, 73 | - | 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) | ||
a Retention time (RT) (min). RSD: relative standard deviation; VOMs: volatile organic metabolites. The results are expressed as mean values (n = 3; RSD < 20%). b Kovats index (KI) relative n-alkanes (C8 to C20) on a BP-20 capillary column. c Kovats index (KI) relative reported in literature for equivalent capillary column [18,20]. - Not detected; DDMP: 2,3-Dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one.