## Results and Discussion  
### HS-SPME Optimization and Validation  
Extraction efficiency in SPME not only depends on the polarity and thickness of the stationary phase, but is also affected by other variables such as agitation, time, temperature, or the addition of salts to the sample [28]. As a first step, we compared the performance of different fiber coatings for the extraction of volatile compounds from strawberry: polydimethylsiloxane (PDMS), polyacrylate (PA), polydimethylsiloxane/divinylbenzene (PDMS/DVB), carboxen/polydimethylsiloxane (CAR/PDMS), and divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS). For this purpose, generic extraction conditions were applied based on our previous experience and literature: equilibration time, 15; extraction time, 30 min; extraction temperature, 50 \({}^{\circ}\)C; weight of sample, 10 g; weight of background electrolyte, 10%. The extraction efficiency was evaluated in terms of the total number of compounds extracted and their peak areas. The results evidence that, although different fibers were best suited for the extraction of certain compounds, the best overall extraction efficiency was obtained by using DVB/PDMS coatings, in line with previous studies [29; 30; 31]. Furthermore, we also checked that the HS volume did not have a significant influence on the extraction process if maintained in the range 30-50% of the total volume, thus providing suitable reproducibility and extraction recovery. To investigate the effect of other variables in the adsorption/desorption process, a two-level screening fractional factorial experimental design (2\({}^{(7-3)}\)) with three replicates in the center point was carried out. This screening experimental design evidenced a significant positive influence of sample weight and adsorption temperature on the extraction efficacy, which reached a plateau at the higher experimental conditions tested in this study. Accordingly, these parameters were subsequently optimized by using a central composite experimental design (CCD, with \(\alpha=1.414\)). To this end, sample weight and extraction temperature were tested in the ranges 6.5-12.5 g and 30-80 \({}^{\circ}\)C, respectively. All the other variables were kept at levels providing the best responses according to the Pareto diagram (Figure 1): equilibration time, 30 min; extraction time, 20 min; desorption time, 5 min; desorption temperature, 240 \({}^{\circ}\)C; weight of salt, 10%. By applying the response surface methodology, the estimated optimum values for extraction temperature and sample weight were 65.4 \({}^{\circ}\)C and 11.4 g, respectively (Figure 2). The adequacy of the model was evaluated by using the coefficient of determination (R\({}^{2}\)), and assessing the regression fitness. The model showed statistical valid fitness (\(p<0.05\) for the regression F test) and non-significant lack of fit (\(p>0.05\)). In addition, the coefficient of determination was estimated as 0.419. To assess the precision of the extraction method previously optimized, nine independent extractions were carried out from the same batch of fruit puree at the optimum conditions. The extracts were  
Figure 1: Pareto diagram showing the effect of factors investigated on the extraction efficiency. Figure 2: Response surface plots showing the effect of temperature and sample weight on the extraction efficiency according to the central composite design (CCD). prepared in three non-consecutive days, and replicated three times within each day. Repeatability was evaluated as the relative standard deviation of the ANOVA within-condition variance (S\({}^{2}\)W) considering the different days as the variation source. On the other hand, intermediate precision was evaluated from between-condition variance (S\({}^{2}\)B) and within-condition variance (S\({}^{2}\)W): S\({}^{2}\)R = S\({}^{2}\)W + S\({}^{2}\)B [32]. The results evidence satisfactory repeatability, in the range 3.2% (ethyl butanoate) to 13.5% (hexanal), and intermediate precision, in the range 6.3% (geraniol) to 22.1% (hexanal).