Determination of free DOPA and 3-O-methyl-DOPA in human plasma by high-performance liquid chromatography with electrochemical detection

A procedure was devised for the determination of the unconjugated non-protein-bound fraction of 3,4-dihydroxyphenylalanine (DOPA) and 3-O-methyl-DOPA (3-OMD) in plasma using a reversed-phase liquid chromatographic system coupled with electrochemical detection. Sample preparation involves rapid isolation of the unbound drugs from the drug-protein complex by ultrafiltration through a membrane with a molecular weight cut-off of 10 000 dalton. One chromatographic run requires less than 10 min. The relative standard deviation is <3% for the within-assay imprecision and <4% for the between-assay imprecision. The detection limits for DOPA and 3-OMD are 0.2 and 1.3 ng/ml, respectively.