Sepra™ C18-E (50 µm, 65Å, bulk packing) (Phenomenex, Torrance, USA) was washed twice with water and isopropanol (IPA), firstly at 4:1 (v/v), then at 1:4 (v/v) in Falcon tubes. The sorbent was further washed and conditioned three times with the extraction solvent, and the supernatant (after centrifugation) was decanted and discarded after each step. The solvent selected after the development of the protocol was a mixture 1:1 (v/v) of acetonitrile (MeCN) and methanol (MeOH).