During the whole sample preparation procedure, the samples were kept on ice. At first, 200 μL urine or plasma were mixed with 20 μL internal standard mix (Table ) and 20 μL of the 13 C-labeled yeast metabolite extract. For the experiment to determine the best solvent/column combination no internal standard was used. Therefore only 40 μL H2O were used instead of the internal standard mix. The samples were then vortexed. Afterwards, the samples were mixed with 760 μL of extraction solvent (ACN: MeOH (1:1, v:v)). After thoroughly vortexing and sonication on ice (10 min), the samples were put at -20°C for 2 h and centrifuged at 18,000 x g and 4°C (10 min) and 960 μL of the supernatant was transferred to a new tube. Then, the samples were evaporated in a vacuum concentrator (Labconco). The residues were reconstituted in 192 μL solvent , vortexed and centrifuged at 4°C for 10 min. Finally, the supernatants were transferred to HPLC vials and stored at -80°C until analysis. Moreover, matrixmatched calibration standards for urine and plasma were prepared by reconstituting matrix extracted according to the sample preparation protocol with respective solvent standard solutions. SRM3672 was additionally treated with βglucuronidase/arylsulfatase enzymes from Helix pomatia for 12 h at 37°C prior to extraction as the NIST certified reference values are given for deconjugated samples. This additional step was not performed for the other samples because the heat treatment would have disturbed measurements of endogenous metabolites. A non-deconjugated sample was prepared for the SRM material for comparison as well.