High-performance liquid chromatography with amperometric detection of plasma L-3,4-dihydroxyphenylalanine in Parkinsonian patients.

An assay for the quantitative estimation of L-3,4-dihydroxyphenylalanine in human plasma has been developed, using alpha-methylnoradrenaline as internal standard and ion-pair reversed-phase (C8), isocratic high-performance liquid chromatography with amperometric detection. A citrate-phosphate buffer, pH 3.1, containing 2 mM EDTA disodium salt, 6.5 mM 1-octanesulphonic acid and 14% v/v methanol, provided good separation of the analytes from each other and from the carbidopa present in the plasma of Sinemet-treated Parkinsonian patients. The sensitivity, selectivity and precision of the method were found to be sufficient to enable its routine use for pharmacodynamic studies.