After this period of time, samples were removed and allowed to cool to room temperature. A pH-value of about 7.4 was adjusted by addition of 12.5 ml of 0.1 M aqueous dipotassium hydrogen phosphate and 1.25 ml of 0.8 M aqueous sodium hydroxide. After this step, 100 μl of a freshly prepared solution of the internal standard 4-nitrobenzaldehyde semicarbazone (20 μg ml−1 in DMSO) were added. The homogenised sample solution (about 24 ml) was filtered through a type 589 paper filter (Schleicher & Schüll, Dassel, Germany) and afterwards washed with 3 ml of water. The filtrate was applied to a 200-mg LiChrolut® EN SPE column (Merck, Darmstadt, Germany) preconditioned with 3 ml of ethyl acetate, followed by 3 ml of methanol and 5 ml of water. After sample loading the column was washed with 2.5 ml of water and dried by sucking through air until the sorbent completely changed its colour from brown to orange. Elution of the analytes was achieved with 3 ml of ethyl acetate. The eluate was evaporated under a stream of nitrogen and the residue redissolved in 250 μl of mobile phase. A 50-μl volume of this solution was injected into the LC–MS–MS system. During the whole procedure, direct exposure of the sample solutions to light was avoided to prevent any photodegradation.