Forty-nine chemicals were annotated (Level 1-3) in the polar fraction of the serum samples, with three statistically significant results (1,7-dimethyluric acid, pipecolic acid, and amantadine). After data normalization, the PCA plot (Figure ) displayed all the QC samples with a tight clustering. This indicates that the instrument variation was effectively corrected, affirming a good system stability and reliability of the results.