## 3 Results and Discussion
### Extra-Virgin Olive Oil Complex Validome by GC \(\times\) GC Fingerprinting  
The chemical complexity of the olive-oil volatilome can be effectively described by the concept defined by [PERSON], known as chemical dimensionality [29], which was introduced to describe the degree of order/disorder that can be achieved in multidimensional separations. Volatiles in olive oil are generated from multiple chemical reactions, mainly promoted by endogenous or exogenous enzymes, that occur in olive primary metabolites during fruit ripening and, later, in post-harvest and processing stages. In addition, storage and shelf-life may add additional complexity, resulting in thousands of volatiles that belong to different chemical classes and differ in their polarity, volatility and concentration. High resolution separations and orthogonal detection by mass spectrometry are fundamental for the accurate fingerprinting of volatiles. In addition, the possibility of obtaining structured separation patterns for chemically correlated compounds is of great help; analyte identification can be confirmed by observing analyte relative elution, while for unknowns, information about their relative polarity and volatility can be reliably hypothesized because of the multiple retention mechanisms used by the technique. Figure 1A shows the 2D pattern of the PGI Toscano EV oil, which was taken as a reference sample for the study. The number of detectable 2D-peaks, over a signal-to-noise ratio (SNR) threshold of 50, is about 1500, and reliable identification was possible for 114 of them by matching \({}^{1}\)D \(I^{T}\) and MS spectra with those collected in commercial and in-house databases [30; 31]. Of the most important classes of informative volatiles, compounds formed from lipoperoxide cleavage, also referred to as the lipoxygenase (LOX) signature (Figure 1B), are fundamental for the definition of fresh-green and fruity notes, which are considered positive attributes. Of theC6 unsaturated alcohols and aldehydes, hexanal was connoted by green apple and grassy notes, (Z)-3-hexenal had green and grassy odor, (E)-2-hexenal was described as bitter almond and green, (Z)-3-hexenol and (E)-3-hexenol had both green notes, (E)-2-hexenol evokes green grass and leaf odors, while (E,Z)-2,4-hexadienal had a green odor. All of these compounds were formed from linoleic and linolenic acid oxidative cleavage, as promoted by lipoxygenase (LOX) and hydroperoxide lyase (HPL) pathways [32]. Figure 1C illustrates the homologous series of linear saturated and unsaturated aldehydes, together with a few ketones that were most likely formed by non-enzymatic hydroperoxide cleavage. This last group of analytes generally provides information on shelf-life evolution [33]; increasing concentrations of potent odorants of this class bring rancid and fatty notes. Heptanal, octanal, and nonanal, although possessing different odor potencies, had fatty and waxy notes, decanal and undecanal were described as waxy and fatty, while the series of (E)-2-unsaturated aldehydes (i.e., from (E)-2-heptenal to (E)-2-Decenal) had odors that evoked apple and green leaf, up to fatty and tallowy notes for the higher homologues. The enlarged area of Figure 1D shows the retention region of a group of branched unsaturated hydrocarbons, eluting later in the 2D. They were identified by [PERSON] et al. [34] in olives at early stages of ripening. They were 3,4-diethyl-1,5-hexadiene (RS + SR), 3,4-diethyl-1,5-hexadiene (meso), (5_Z_) and (5_E_)-3-Ethyl-1,5-octadiene, (_E_,_Z_)- and (_E_,_E_)-3,7-decadiene and (_E_)-4,8-Dimethyl-1,3,7-nonatriene [12; 34]. This chemical complexity can also be explored by simple datapoint feature fingerprinting [35]; this pointwise approach, also explored by [PERSON] et al. [9] in a study focused on olive oils from different cultivars, enables point-by-point, or pixel-by-pixel, chromatogram comparisons to be performed. In a GC \(\times\) GC-TOF MS chromatogram, every datapoint corresponded to a detector event, i.e., a single MS spectrum.