Automated determination of levodopa and carbidopa in plasma by high-performance liquid chromatography-electrochemical detection using an on-line flow injection analysis sample pretreatment unit.

An automated analytical procedure is described for the parallel determination of L-3,4-dihydroxyphenylalanine (levodopa, L-dopa, LD) and the analogous hydrazine compound carbidopa (CD) in dog plasma by ion-pair high-performance liquid chromatography with electrochemical detection (HPLC-ED). After deproteinization of the plasma samples with perchloric acid the catecholamines were extracted from the supernatant by adsorption on a small column filled with alumina. The extraction and redissolution were automatically performed in a flow injection analysis unit (FIA) coupled to the HPLC system. The performance of the whole system was tested on dog plasma samples including specimens taken after oral administration of the anti-Parkinsonism drug Duellin, which is a combination tablet of levodopa and carbidopa.