A 20 µl portion of internal standard ( 13 C-OTA) and 20 µl of β-glucoronidase were added to 1 cm 3 of serum. The samples were then incubated for 18 hours at 37 o C. After the incubation, 1 cm 3 of MeOH and 1.96 cm 3 of ACN were added to the mixture. Subsequently, the solution was vortexed for 2 minutes and centrifuged in a centrifuge for 10 minutes at 7,000 rpm. A 3 ml portion of the supernatant was transferred to a 50 cm 3 tube and evaporated in a stream of nitrogen. After evaporation, the samples were dissolved in 1 cm 3 of MeOH, first for 3 minutes in an ultrasonic cleaner and subsequently for 5 minutes in a shaker. In the next step, 25 cm 3 of PBS was added to the solution. The solution was quantitatively transferred to an OchraPrep immunoaffinity column (R-Biopharm Rhône LTD). After the sample passed through the column, it was washed with 20 cm 3 of distilled water and air dried. OTA was eluted by 1.5 cm 3 of MeOH:CH 3 COOH (98:2) into a 2 ml tube. Subsequently, it was evaporated in a stream of nitrogen at 40 o C. Immediately before LC -MS/MS analysis, the samples were dissolved in 150 µl of H 2 O:MeOH (7:3) mixture.