Positive and negative electrospray full scan MS1 chromatograms of Belgian endive ( C. intybus ) extract. Protocol: Fresh Belgian endive heads are frozen by immersion in liquid nitrogen, and subsequently crushed to a fine powder in a pre-cooled blender. The powder is stored at -80 ˚C in plastic tubes (Falcon) tubes until use. For each sample 300 mg of crushed frozen leaves is weighed in a pre-cooled 2 mL plastic tube (Eppendorf). These tubes can also be stored at -80 ˚C for a few days before use. Care is taken to prevent thawing of the frozen material up until this point, by pre-cooling of all materials which comes in contact with the plant powder. Failure to do so causes a brownish discoloration in the subsequent steps which is symptom of oxidation.For the extraction, 1.4 mL of extraction solvent is added to each small plastic tube. Extraction solvent is prepared by adding 1 mL of formic acid and 100 µL of acetonitrile containing 20 mg·mL-1 of α-santonin to 1 L of HPLC water Extraction is performed under ...