**2. Materials and Methods**

#### *2.1. Reagents and Oil Samples*

Glucose (≥99.5%), lactic acid (90%), formic acid, sulfuric acid (95.0–98.0%), methanol and acetonitrile of HPLC grade, methanol and ethanol of analytical grade, hydroxytyrosol, and tyrosol were purchased from Sigma-Aldrich (Sigma-Aldrich Co. LLC, St. Louis, MO, USA). Ultrapure water was obtained from an Elga Purelab Option R system (Veolia Environnement S.A., Paris, France). Extra virgin olive oil (EVOO) samples (*n* = 26) were obtained from producers and research laboratories. They differed by geographical origin (different Italian regions), cultivar, olive maturity, and extraction technology.
