## 2 Materials and Methods
### Mass Spectrometry Analysis  
The phytochemical characterization of LO obtained from the mastic tree (_Pistacia lentiiscus_) was achieved by means of a multimethodological approach based on both direct infusion Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with an electrospray ionization (ESI) source and gas chromatography (GC) coupled with a single quadrupole mass spectrometer. The combination of these two techniques provides high metabolome coverage, revealing a wide spectrum of compounds, from (semi) polar (ESI-FT-ICR MS) to less polar ones (GC-MS). In particular, the ultra-high resolving power, sensitivity and mass accuracy of the FT-ICR MS allows rapid chemical fingerprinting of complex mixtures to be achieved, such as foodstuffs, plant extracts, and essential oils [32]. Herein, a small amount (500 \(\upmu\)L) of LO was filtered through a syringe filter (0.45 \(\upmu\)m hydrophobic polypropylene, Acrodisc, Sigma-Aldrich, Milan, Italy) to remove particulates and debris. A sample of 250 \(\upmu\)L of filtrate solution was diluted in 1 mL methanol/acetonitrile (50/50, \(v/v\)) and vortexed for 3 min; then, the solution was further diluted in the ratio of 1:1000 in methanol, to reach a final 5-10 \(\upmu\)M concentration. To enhance ionization, formic acid and ammonia, both at the final concentration of 0.1% (\(v/v\)), were added in positive and negative ionization mode, respectively. The diluted solution was directly infused in the electrospray source hyphenated with a high-resolution mass spectrometer (Bruker BioApex 4.7 T FT-ICR, Bruker Daltonics GmbH, Bremen, Germany). The analyses were performed in both ionization modes (ESI(+), ESI(-)); the syringe flow rate was set at 120 \(\upmu\)L/h; the spectra were acquired in the range of 100-1000 \(m/z\); 200 scans were collected and 3 technical replicates recorded. All raw data were pre-processed on the Bruker Compass DataAnalysis software (3.4 version) for mass calibration and peak picking. The \(m/z\) values with a cut-off S/N of four were submitted to a home-made script for the annotation, wherein each \(m/z\) value was first associated to a univocal molecular formula and then to one or more metabolite isomers. The script has carried out the assignment by enquiring different databases; for each \(m/z\) value, protonated ([M + H]\({}^{+}\)) and alkali metal adducts ([M + Na]\({}^{+}\), [M + K]\({}^{+}\)), in positive mode, and deprotonated ions ([M - H]\({}^{-}\)) and chloride adducts ([M + Cl]\({}^{-}\)), in negative mode, have been considered. In addition, the analysis revealed the presence of some cationic species that have been detected and indicated as M\({}^{+}\). A comprehensive list of putative compound annotations, together with theoretical and experimental adduct \(m/z\), neutral mass, and peak intensity are available in Table S1 (ESI(+)) and Table S2 (ESI(-)). Most of the entries have been annotated considering a mass accuracy lower than \(\pm 3\) ppm. Collision-Induced Dissociation (CID) experiments have been carried out as additional analyses to confirm the identity of the candidate metabolites by comparing the experimental fragmentation pattern and reference spectra available in the literature or the database. Qualitative and semi-quantitative analyses were furnished by graphical tools, such as van Krevelen diagrams, histograms of relative abundance distribution and donut chart of elemental composition, so giving an overview of the elemental ratio composition and molecular density of several classes of metabolites. Meanwhile, GC-MS is the most suitable technique to assess the Volatile Organic Compound (VOC) profile of a complex matrix, i.e., food, essential oils [33]. Herein, vapors (headspace--HS) in equilibrium with the liquid oil at a controlled temperature were collected through two different approaches.