**4. Materials**

#### *4.1. Chemicals, Reagents and Materials*

Methanol (MeOH), acetonitrile (AcN), and water for liquid chromatography (LC) mobile phase (HPLC grade) were purchased from Merck (Darmstadt, Germany). Ammonium formate and formic acid were acquired from Fluka (Milan, Italy). Sodium chloride and C18 were provided from Sigma–Aldrich (Milan, Italy). Syringe filters with polytetrafluoroethylene membrane (PTFE; 15 mm, diameter 0.2 μm) were purchased from Phenomenex (Castel Maggiore, Italy). Analytical standard of Enn B1 ( >95% HPLC purity) was obtained from Sigma–Aldrich (Milan, Italy). A stock solution (1 mg/mL in MeOH) was prepared and working standard solutions were built by serial dilution of the stock and stored at −20 ◦C.
