3.1. Essential Oils Characterization
Uncoloured, yellow and blue liquid oils are the result of the hydrodistillation of the aerial parts, lives and fruits of the different used species. Overall, numerous compounds were identified for every sample, representing from a minimum amount of 88.97% to a maximum of 99.73% of the total composition. One hundred eighty-one compounds were detected and quantified by GC and GC/MS methods in the studied oils. All these constituents were identified on the basis of their mass spectra, retention indices and/or by comparison with authentic compounds [22
]. The analytical data are the average of three injections. Each one of the eleven samples shows a very peculiar chromatographic profile. The complete characterization of the essential oils is reported in Table S1 (Supplementary File)
Among the analyzed plants we observed that the fruits of Juniperus oxycedrus L. ssp. macrocarpa (Sibth & Sm.), collected in December, were characterized by an essential oil in which the main constituent is represented by a very higher percentage of pinene, 77.63% on 91.54% of identified compounds. With the exception of germacrene D (3.5%), all the other constituents are present in concentration <1%. No other essential oil among those investigated exhibits a similar chromatographic profile.
From the aerial parts of Achillea maritima (L.) Ehrend. & Y.P. Guo an essential oil with a very stable dark blue colour was obtained, in which some constituents in high concentration could be identified. The blue colour is due to azulenes, in particularly to chamazulene (0.38%). Yomogi alcohol reached the concentration of 22.83%, artemisia alcohol 17.18% and camphor 20.82%. Other remarkable constituents were camphene (5.38%), artemysil acetate (5.28%) and crysantenone (3.38%). With the exception of camphor, all the other mentioned compounds were not found in the other analysed oils.
We also analysed a commercial essential oil of Lavandula officinalis L. that has a concentration of 32.01% of linalool, 28.65% of linalyl acetate, 7.4% of camphor and 5.07% of linalyl acetate.
The essential oil obtained from flamed of Citrus limon var. pompia Camarda fruits was colorless and characterized by the presence of 88.23% of limonene followed by a 2% of myrcene; typical constituents of the oils of Citrus species, such as neral, geranial and geraniol (1.15%, 1.56% and 1.23% respectively), were also identified. All the other constituents were present in very low concentration.
The essential oil extracted from M. communis leaves had as principal constituents α-pinene (52.72%), 1,8-cineole (16.11%) and limonene (6.53%).
The aerial parts of Santolina insularis (Fiori) Arrigoni gave a yellow essential oil rich in artemisia ketone (24.61%) and β-phellandrene (18.87%), which were the main constituents. Other constituents in high concentration were santolina triene (9.32%), found only in this essential oil, 1S-pinene (7%) and myrcene (5.15%).
Thymus capitatus (L.) Hoffmanns. & Link, as many Thymus species, was characterized by a high carvacrol concentration (57%); other important constituents were o-cymene (8.65%), γ-terpinene (6.92%), 3.7% of myrcene and α–thujene.
Four samples of Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo and Giusso were collected in four stations located in South-West of Sardinia. The four essential oils obtained were very different each other; in particular the samples coming from Station 1 (Iglesias) and Station 2 (Carbonia) were characterized by an unusual concentration of γ-curcumene (more than 28%). The essential oil obtained from plants collected in Station 3 (Portoscuso) presents many constituents with concentration among 1 and 2% and the main constituents are represented by γ-curcumene (12.22%), linalool (11.47%), 5-eudesmen-11-ol (10.35%) and nerolidol (5.07%). The oil from station 4 (Oristano) showed the chromatographic profile expected for this species growing in middle and North Sardinia and is very rich in geranyl acetate (33.6%).