Simultaneous assay of 3,4-dihydroxyphenylalanine, catecholamines and O-methylated metabolites in human plasma using high-performance liquid chromatography.

We devised a procedure for the simultaneous determination of 3,4-dihydroxyphenylalanine, catecholamines and O-methylated metabolites using a reversed-phase liquid chromatographic system. Detection is achieved by an electrochemical detector and a fluorescence detector connected in series. Sample preparation is kept to a minimum, and involves precipitation of proteins with trichloroacetic acid and perchloric acid, and subsequent neutralization, thus omitting the commonly adopted adsorption step. Chromatographic peaks were identified on the basis of retention behaviour and the ratio of responses at several oxidation potentials. The method was applied to the quantitative determination of 3,4-dihydroxyphenylalanine, catecholamines and O-methylated metabolites in human plasma.