## 4 Conclusions  
The quality control of EO to highlight the presence of synthetic \"naturally identical\" substances or of less expensive EO can successfully be carried out by evaluating the chemical composition in terms of the normalised percentage area or true quantitation of the diagnostic markers to be compared with the reference data reported in pharmacopeia or ISO norm monographies. Most of the EO samples analysed in routine quality controls comply with the reference data (i.e., pharmacopoeia, ISO norm and in-house built reference profiling). Conversely, borderline EO samples require a more accurate evaluation to confirm or exclude their adulteration. This study shows different approaches on how to deal with a successful quality control of borderline EO samples by using enantiomeric recognition and an absolute quantitative analysis of the selected marker compounds as a complement to the normalised relative abundances, thus making possible to highlight a number of EO adulterations with examples from real world samples. The approaches here adopted were based on gas chromatography, which is the technique of choice to characterise an EO. These methods afford to detect simultaneously both the presence of compounds deriving from different (cheaper) EO with a single GC run, to monitor the addition of synthetic racemic compounds by enantioselective and EO dilution with vegetable oil(s). Moreover, these approaches are very useful in routine quality control, because they do not require extra statistical elaboration, and analyses can be carried out using fast methods with the adoption of narrow bore GC columns, thanks to the repeatability of the separations ensured by the method translation software [31; 32]. Adulteration with vegetable oils cannot be revealed using the above approach and requires an absolute quantitative analysis. This method of course requires reference concentration values obtained from the analysis of a consistent number of certified genuine EO samples. A quantitative analysis is often considered to be a complex time-consuming procedure; however, as clearly shown in this study, the quantitation of a limited number of selected markers is often sufficient to highlight an adulteration with vegetable oil(s); that is, the number of required analyses is rather low. Direct methods based on spectroscopic methods have also been developed with success to deal with this problem: they include fluorescence spectroscopy [9], infrared spectroscopy [11] and Raman spectrometry [10]. These methods, however, generally require the use of multivariate statistical elaborations (principal component analysis and independent component analysis) or the building up of artificial neural networks, thus implying a further step of data processing. Very recently, [PERSON] et al. reported a method based on NMR spectroscopy without a further statistical step not only able to detect the presence of an adulteration with vegetable oils but, also, to identify the added adulterant through its \({}^{13}\)C-NMR fingerprint [12]. The availability of separation and direct methods (in particular, NMR) is an effective step ahead to monitor EO adulterations, since it is now possible to define their quality both in terms of the characteristic qualitative and quantitative marker composition and detection and identification of adulterants of low economic value. In conclusion, the approaches adopted in this study, in combination with the methods based on NMR spectroscopy, open up a concrete possibility of identifying unambiguously EO adulterations by vegetable oil in quality control laboratories. The following are available online: Tables S1-S3. EC% of all enantiomeric compounds in the genuine essential oils investigated. Table S4. The equations for the calibration curves that were obtained with an external standard approach and used to quantitate the marker compounds, together with their correlation coefficients and the selected range of concentrations. Figure S1.