The method for acidified plasma was established to prevent the degradation of potentially formed acyl-glucuronides in plasma. The method for analysis in non-acidified plasma had two calibration ranges, a low calibration from 1.00 to 1,000 ng/mL and a high calibration range from 20.0 to 20,000 ng/mL. S3I-201 Following protein precipitation with acetonitrile containing the stable labeled internal standard and centrifugation, 10 μL of the diluted sample

was injected onto the analytical column. Solid-phase extraction was used for processing of acidified plasma samples. Plasma samples were fortified with the labeled internal standards and applied under a gentle vacuum onto 96-well SPEC C18 AR SPE plates (Agilent Technologies, Palo Alto, CA, USA). After washing with 0.1 % formic acid and a mixture of 0.1 % formic acid/methanol (9:1, v/v) samples were eluted with 200 μl methanol, diluted with 0.1 % formic acid (1:1, v/v), mixed, and aliquots of 10 μl were injected

onto the chromatographic system. The chromatographic system consisted of a Rheos 2200 pump (Thermo Fisher find more Scientific, Waltham, MA, USA), an analytical column (method for non-acidified plasma: Atlantis d C18, 2.1 × 20 mm, 3 μm (Waters, Milford, MA, USA); method for acidified plasma: Zorbax XDB-C18, 2.1 × 50 mm, 5 μm (Agilent Technologies, Palo Alto, CA, USA), and an autosampler (PAL; CTC Analytics, Zwingen, Switzerland). For the chromatographic separation, the solvents were solvent A (0.1 % formic acid) and solvent B (0.1 % learn more formic acid from in acetonitrile). The flow rate was set to 0.4 mL/min. A gradient was used in which solvent B was held at 10 % for 0.2 min and that increased linearly up to 90 %

within 0.8 min. Mass spectrometric analysis was performed with a triple quadrupole mass spectrometer TSQ Quantum (Thermo Fisher Scientific, Waltham, MA, USA) operating in positive electrospray ionization mode with capillary temperature at 350 °C and spray voltage at 4.0 kV. The inclusion of quality control samples throughout the complete study assured method integrity. An analytical run was accepted when at least two-thirds of the quality control samples (analyzed in duplicate) were within ±15 % of their nominal value and when not more than 50 % of the quality control samples at the same concentration were outside this limit. In non-acidified plasma, the accuracy and precision of the quality control samples were 88.0–100.7 and 2.3–18.0 % in the low calibration range and 102.3–105.3 and 1.8–6.2 % in the high calibration range. In acidified plasma, the accuracy and precision of the quality control samples were 99.1–110.7 and 2.0–8.0 %, respectively. 2.