2.3 Sample preparation
A 600 μL acetonitrile and 10 μL IS (TGC-d9, 500 ng/mL) was added in 200 μL serum or CSF. After 1 min vortex and 4000 rpm centrifugation for 5 min, 300 μL supernatant was extracted from samples and then processed by 0.22 μm filter. A 10 μL filtrate was injected into the UHPLC-MS/MS system for analysis.