Sample preparation. Samples were prepared by liquid-liquid extraction according to a validated method with slight optimization. Briefly, 150 µL plasma aliquots were (on ice) mixed with 5 µL of antioxidant solution (0.4 mg/mL BHT in MeOH: 0.4 mg/mL EDTA in water, 1:1 v:v), 40 µL of 0.4 mg/mL BHT solution in MeOH, 150 µL of a 0.2 M citric acid and 0.4 M disodium hydrogen phosphate buffer in water, and 1 mL of BuOH:MTBE (1:1 v:v). After resting for 20 min on ice, they were mixed in a bullet blender (2 min, highest speed) and centrifuged (10 min, 4 °C, 15700 g). A volume of 850 µL of the upper organic phase was transferred to a 1.5 mL Eppendorf tube and dried overnight in a SpeedVac (ThermoFisher Savant) at 30 °C. The dried samples were reconstituted in 50 µL of ice-cold injection solution (90% MeOH, 10% Milli-Q, 100 nM CUDA) to prepare blank plasma or in 50 uL of a combination of the injection solution, (deuterated) standard mix and 0.4 mg/mL BHT in MeOH. Samples were vortex mixed and centrifuged (10 min, 4 °C, 15700 g) prior to being transferred to an autosampler vial with an insert.