2.2Standards and quality controls
Stock solution of TGC (1mg/mL) was prepared by dissolving 10 mg TGC in 10 mL deionized water, which was subsequently diluted to a series of concentrations of 0.05, 0.20, 0.50, 1.00, 2.00, 5.00, 10.0 and 20.0 μg/mL as standard solution. A 20 μL of standard solution was added into 180 μL blank serum or CSF to make the serum or CSF calibration at concentration of 5, 20, 50, 100, 200, 500, 1000 and 2000 ng/mL. Three concentration levels (10, 250 and 1800 ng/mL) of quantity control (QC) samples were prepared for analysis of serum and CSF samples in the same way. Standards and QCs were stored at 4°C for no longer than 30 min or frozen at −80°C for long-term storage.