then mixed with 2 mL of HCl in methanol and 1 mL of boron trifluoride solution (Sigma–Aldrich, St. Louis, MO, USA), heated again in a 90 C water bath for 15 min, and then cooled again.
Next, 3 mL of a saturated NaCl aqueous solution and 4 mL of n-hexane were added, and the samples were mixed thoroughly. The upper liquid layer was transferred to a 4-mL amber glass vial, sealed using Parafilm, and stored at 20 C until analysis was performed.