terpenes/terpenoids and phenolic compounds (i.e., phenylpropanoids), that derive from the mevalonate/methyl erithrytol and shikimic acid pathways, respectively.  
The chemical composition of an EO is usually expressed in the literature in terms of the relative percentage abundance (i.e., % area) or normalised percentage abundance (i.e., norm % areas) [3]. Only a few papers have reported the true quantitation of EO, as determining this is considered difficult and time-consuming.  
The quality control of EO is necessary to guarantee their safe use, as well as to detect adulterations and fraud. Unfortunately, the adulteration of EO is not uncommon along supply chains, thus generating concerns in the EO industry. EO can often be adulterated via the addition of cheaper EO (e.g., sweet orange added to bitter orange, corn mint added to peppermint or lavandin added to lavender) or via the addition of cheap synthetic materials (e.g., synthetic linalool and linaly acetate added to bergamot EO) [4]. This type of adulteration can be detected quite easily via the determination of the normalised percentage areas of selected markers. Moreover, since biosynthesis in plants is stereochemically guided and terpen/terpenoids are generally chiral compounds with a specific enantiomeric composition [5; 6], chiral marker compounds become diagnostics for detecting the adulteration of essential oils via the addition of synthetic volatile compounds. Enantiomeric recognition is therefore also necessary to improve the quality control and uncover fraud and adulteration via the addition of cheap synthetic materials or volatiles from other sources to EO [4].  
Dilution with vegetable oils, resulting in a reduction in scent, is another type of EO adulteration. Vegetable oils are selected, as they are relatively cheap and because they present a density and texture that are similar to those of EO [4]. This type of adulteration is quite obvious, as it modifies neither the qualitative composition of the EO nor the relative percentage abundance of the marker compounds. However, the dilution of the final product interferes with the EO sensory and biological properties, in addition to committing commercial fraud. In this case, the normalised percentage area is no longer diagnostic, and a true quantitative analysis is required, provided that acceptable reference quantitative data are available.  
To the best of the authors' knowledge, several papers describing strategies for revealing EO adulterations that occur via the addition of cheaper EO or synthetic compounds have been reported in the literature and have recently been reviewed [4; 7; 8]. Conversely, there are few papers that describe approaches to reveal the addition of vegetable oils to dilute EO [9; 10; 11]. The most applied technique is infrared (IR) spectroscopy coupled with a multivariate analysis. However, the precise identification of vegetable oil is often difficult due to the signal overlap of similar molecules [11]. Very recently, [PERSON] et al. introduced a new method based on \({}^{13}\)C-NMR spectroscopy to recognise adulterant vegetable oils in EO; the method does not require additional data elaboration with a multivariate analysis [12].  
This study evaluates and compares different approaches to detect adulterations of three representative EO (i.e., bergamot, lavender and tea tree EO)--in particular, the determination of the normalised percentage areas and/or the enantiomeric composition of selected markers and the true absolute quantitative analysis. These three EO were selected due to their global market impact, which includes a constant increase both in terms of production and worldwide market share [13]. Both conventional and chiral GC analyses were performed, and the latter was combined with HS-SPME sampling to avoid damage to the chiral column degradation due to non-eluted residues of vegetable oil.