The sample was prepared under the yellow light with a wavelength longer than 420 nm (ultraviolet light was removed). Sample preparation involved a single step of protein precipitation. Aliquots of human plasma (100 L), IS solution (10 × 2 g/mL), methanol (5 L), 10% formic acid aqueous solution (5 L) and acetonitrile (200 L) were added to a 1.5 mL polypropylene centrifuge tube. The mixture was swiftly vortexed for 10 s then centrifuged at 15,000 × g for 10 min. The supernatant was transferred to clean vials for sample injection.