0.45 m cellulose acetate membrane

The plant material (Cinchonae cortex Ph.Eu.) was finely pulverized in a mill and 150 mg were extracted following a published protocol [6 ]. Extraction solvent was a methanol/0.1 M NaOH mixture in the ratio 49/1; the samples were extracted three-times with 10 ml of this mixture by sonication (Bandelin Sonorex, Berlin, Germany) for 20 min each. After each step they were centrifuged for 10 min at 1500g, and the clear supernatant combined in a 50 ml volumetric flask. Then the latter was filled to volume with the extraction solvent. Sample solutions were membrane filtered right before analysis (0.45 µm cellulose acetate membrane, VWR, Vienna, Austria) and injected in triplicate. If stored at 4 °C sample and standard solutions are stable for at least 2 weeks.