γ terpinene

Pure constituents of the EOs were purchased from Sigma-Aldrich [β-pinene (99%), farnesene (a mixture of isomers), limonene (97%), ocimene (a mixture of α and β isomers), p-cymene (99%), γ-terpinene (97%), terpinolene (85%), St Louis, MO, USA], and Merck (myrcene; Darmstadt, Germany).

α-pinene, camphene, β-pinene, D-limonene, cis-β-ocimene, trans-β-ocimene, γ-terpinene, α-terpinolene, (Z)-3-hexenylacetate, (E)-3-hexenylacetate, linalool, methyl salicylate, longicyclene, β-caryophyllene, α-humulene, valencene, (E-E)-α-farnesene, and α-sabinene were purchased from Merck (Isle d’Abeau Chesnes, Saint Quentin Fallavier, France). Stock solutions of each standard were prepared at 10 g/L in ethanol (ethanol absolute, VWR, Fontenay sous Bois, France) and kept at 4 °C. A stock solution of a mixture of all standards was prepared at 100 mg/L in ethanol and stored at 4 °C up to 6 months. For analysis, the stock solution was diluted successively to 1 mg/L in water (LC-MS grade water, Fisher Scientific, Illkirch, France) and then to 50 µg/L. An external calibration was performed with monoterpenes, sesquiterpenes, and green leaf volatile standards prepared in water in the 12.5 μg/L to 1000 μg/L concentration range. The final standard mixture solution was placed into a 20-mL headspace vial, and a stir bar was exposed in the headspace of the vial for 1 h at 50 °C. The VOCs collected on the stir bar were desorbed and analysed according to the method described below. Standards and parameters are listed in Supplemental Table S2.

The GC–MS analysis was carried out with an Agilent 6890N–5973N GC–MS system (Santa Clara, CA, USA) on a sample of M. aquatica EO prepared by dilution to 1:100 with n-hexane. The instrument was operating in the EI mode at 70 eV and using a HP-5MS (5% phenylmethylpolysiloxane, 30 m, 0.25 mm i.d., film thickness 0.1 µm) (J&W Scientific, Folsom, CA, USA) capillary column.
The chromatographic parameters and chromatogram analysis were the same as those reported by Nkuimi Wandjou et al. [27 (link)]. Briefly, the analytical standards of α-pinene, sabinene, β-pinene, myrcene, α-terpinene, p-cymene, limonene, 1,8-cineole, (Z)-β-ocimene, (E)-β-ocimene, γ-terpinene, terpinolene, terpinene-4-ol, α-terpineol, (E)-caryophyllene, and α-humulene were purchased from Merck (Milan, Italy) and used for peak assignments based on retention time and mass spectrum (MS). Moreover, the combination of the calculated linear retention index (RI) and MS was used to confirm the identity of the other compounds. Semi-quantitative values (peak area percentages) were obtained by peak normalization without using correction factors.