## 3 Results and Discussion
### Tocopherol Quantification in Food Matrices
This fact might be due to potential interference in the chromatographic analysis of tocopherols caused by co-extracted compound/s from the fish matrix using the Folch method, which is a more efficient process to extract a wider variety of lipids including polar and non-polar compounds. Mackerel fillet showed values between 0.79 and 1.33 mg/100 g for \(\alpha\)-tocopherol, which are within the range reported by other authors. [PERSON] et al. found 1.59 mg/100 g of \(\alpha\)-tocopherol in Bulgarian mackerel [40], [PERSON] et al. found mackerel to have 1.30 mg/100 g of \(\alpha\)-tocopherol [48], and [PERSON] et al. reported 1.10 mg/100 g of \(\alpha\)-tocopherol [46] when analyzed by FLD, UV-Vis detector, or DAD, respectively. Other studies have shown values around 0.4 mg/100 g of \(\alpha\)-tocopherol evaluated by the same detection systems (UV-Vis or DAD) [49; 50]. In the current work, \(\gamma\)-tocopherol was also found when lipids were extracted from fish with \(n\)-hexane (0.61 mg/100 g); however, other authors have not reported the content of this tocopherol isoform. No significant differences in the concentration of \(\alpha\)-tocopherol were found when using the Folch process (10.1 mg/100 g) and \(n\)-hexane (9.50 mg/100 g) for lipid extraction in almonds, despite the amount of lipids extracted with the Folch protocol being significantly higher than with \(n\)-hexane (52.5 and 44.7 g/100 g dw, respectively). However, a slight but significant increase in \(\gamma\)-tocopherol was noted using the Folch method (1.44 vs. 1.19 mg/100 g). Data regarding the tocopherol content in almonds are highly variable in the literature, considering that tocopherol concentration depends on the genotype, the environmental conditions, the irrigation practices, and the kernel maturation stage, among other factors. For example, values among 1.2 and 84.0 mg/100 g have been reported for \(\alpha\)-tocopherol in almond kernels, which is the most abundant isoform of vitamin E in this food matrix [51; 52]. Several studies have reported the amount of vitamin E in almond oil instead of in almond kernels, showing values in the range of 20-75 mg/100 g oil for \(\alpha\)-tocopherol, and lower values for \(\gamma\)-tocopherol (0.2-7.5 mg/100 g oil) [11; 41; 42; 51]. Whether concentrations of \(\alpha\)- and \(\gamma\)-tocopherols are calculated on an oil basis in the current work (19-20 and 2.4-2.9 mg/100 g oil, respectively), such values are within the above-mentioned ranges. The amounts of \(\alpha\)-tocopherol found in spinach in the current work showed no significant differences between the lipid extraction methods, although the lipid extraction yield was slightly but significantly higher with the Folch method (0.63 g lipids/100 g dw) than using \(n\)-hexane (0.56 g lipids/100 g dw). \(\gamma\)-Tocopherol was only detected when carrying out the lipid extraction with \(n\)-hexane (0.09 mg/100 g fw). A broad signal was observed in the chromatogram of spinach lipids extracted by the Folch procedure at the retention time of \(\gamma\)-tocopherol (Figure S2), but the peak resolution was not considered good enough to proceed with the integration without the risk of overestimating the amount of \(\gamma\)-tocopherol in the sample. The Folch procedure involves the use of chloroform and methanol as extraction solvents with a wider range of polarity than when \(n\)-hexane is used, which probably resulted in the extraction of one or more compounds that overlapped the signal of \(\gamma\)-tocopherol, thus interfering with the proper identification of this tocopherol isoform. Tocopherol values reported in the literature in spinach samples are highly variable. For example, concentrations of \(\alpha\)-tocopherol ranged between 1.6 and 3.9 mg/100 g fw in several studies [22; 44; 53; 54; 55], whereas much lower values (0.01-1.8 mg/100 g dw, converted into <0.001-0.12 mg/100 g fw considering an average moisture content of 93% in spinach) were found in other works [56; 57].