Optimized Olive Oil Phenol Extraction

The SPE method based on hydrophilic interaction was done using MgO-SiOH (15:85; high purity; particle size ranged between 150 and 250 µm)-bonded silica-phase cartridge (6 mL, 1000 mg, Chromabond^®, Florisil^®, Meschery-Nagel, Düren, Germany). A MgO-SiOH-bonded silica-phase cartridge was placed in a vacuum elution apparatus and then conditioned by the successive passing of 6 mL of methanol/water, 6 mL of hexane, and 3 mL of acetonitrile. An aliquot of olive oil (2 g) mixed with 6 mL of hexane and spiked with 1 mL of syringic acid (17 µg mL⁻¹) was applied to the column. The cartridge was washed twice with 3 mL of hexane. The elution conditions were optimized using a Doehlert experimental design to find the best experimental conditions of the three factors affecting the extraction process, which are: the percentage of the methanol in eluent solvent (water/methanol) (%), the volume of the eluent solvent, and the pH of the elution solvent adjusted using an acetate buffer solution: CH₃COO-/CH₃COOH. Finally, the extract was dried under a stream of nitrogen to 1 mL and then filtered through a 0.45 µm filter membrane for a subsequent analysis by HPLC-DAD. Recovery was calculated as follows: $$R\left(\%\right)=Y \left(\%\right)=100\times \frac{\left(A-A_0\right)}{A_S}$$
where $A$ (µg mL⁻¹), $A_0$ (µg mL⁻¹), and $A_S$ (µg mL⁻¹) are the peak areas of syringic acid determined with the spiked OO sample, non-spiked OO sample, and standard (17 µg mL⁻¹), respectively.