Direct detection of tetrahydrobiopterin (BH4) and dopamine in rat brain using liquid chromatography coupled electrospray tandem mass spectrometry

► Tetrahydrobiopterin is an essential cofactor for hydroxylation reaction of cyclic amino acids. ► A novel method is developed to measure intact BH4 and dopamine by using LC-MS/MS. ► This method improves significantly assay performance for BH4 and dopamine in brain. ► It can be applied to many biological studies to directly determine BH4 and dopamine.