Talanta 63 (2004) 1083–1088 Voltammetric determination of L-dopa using an electrode modified with trinuclear ruthenium ammine complex (Ru-red) supported on Y-type zeolite Marcos F.S. Teixeira a,∗ , Márcio F. Bergamini b , Clélia M.P. Marques b , Nerilso Bocchi b a UNESP, Campus do Litoral Paulista, Unidade São Vicente, Pça. Infante D. Henrique s/n, CEP 11330-900 São Vicente, SP, Brazil b Departamento de Qu´ ımica, Universidade Federal de São Carlos, São Carlos, SP, Brazil Received 13 September 2003; received in revised form 29 January 2004; accepted 29 January 2004 Available online 25 March 2004 Abstract The L-dopa is the immediate precursor of the neurotransmitter dopamine. Unlike dopamine, L-dopa easily enters the central nervous system and is used in the treatment of Parkinson’s disease. A sensitive and selective method is presented for the voltammetric deter- mination of L-dopa in pharmaceutical formulations using a carbon paste electrode modified with trinuclear ruthenium ammine complex [(NH 3 ) 5 Ru III –O–Ru IV (NH 3 ) 4 –O–Ru III (NH 3 ) 5 ] 6+ (Ru-red) incorporated in NaY zeolite. The parameters which influence on the electrode re- sponse (paste composition, potential scan rate, pH and interference) were also investigated. The optimum conditions were found to an electrode composition (m/m) of 25% zeolite containing 6.7% Ru, 50% graphite and 25% mineral oil in acetate buffer at pH 4.8. Voltammetric peak currents showed a linear response for L-dopa concentration in the range between 1.2 × 10 -4 and 1.0 × 10 -2 mol l -1 (r = 0.9988) with a detection limit of 8.5 × 10 -5 mol l -1 . The variation coefficient for a 1.0 × 10 -3 mol l -1 L-dopa (n = 10) was 5.5%. The results obtained for L-dopa in pharmaceutical formulations (tablet) was in agreement with compared official method. In conclusion, this study has illustrated that the proposed electrode modified with Ru-red incorporated zeolite is suitable valuable for selective measurements of L-dopa. © 2004 Elsevier B.V. All rights reserved. Keywords: Modified carbon paste; Ru-red/zeolite Y; Voltammetric determination; L-dopa; Pharmaceutical formulations 1. Introduction L-dopa is a chemical substance used in the treatment of patients with Parkinson’s disease, acting efficiently in the ease of symptom. The substance is converted in dopamine by enzymatic reaction (dopa-descarboxilase) compensating the deficiency of dopamine in organism [1]. Several methods for the determination of L-dopa have been described in literature including spectrophotometric [2,3], flow injection systems [4,5], HPLC [6,7] and gas chromatography [8]. As with other catecholamines, L-dopa has electroactive groups. The oxidation process to quinone has been widely studied from an electrochemical point of view [9,10]. The electrochemical oxidation process of L-dopa in neutral solu- tion belongs to an EC mechanism. L-dopa is electrooxidized ∗ Corresponding author. Tel.: +55-13-35699400; fax: +55-16-34697374. E-mail addresses: funcao@dq.ufscar.br, funcao@csv.unesp.br (M.F.S. Teixeira). into dopaquinone and the dopaquinone cyclizes to cyclodopa and can be oxidized by dopaquinone quickly. However, the necessity of high overpotential, for direct L-dopa oxidation, limits its use for practical purposes, due to several side reac- tions which poison the electrode surface. On the other hand, there are few papers on the determination of L-dopa in phar- maceutical formulations using modified electrode [11,12]. Most of the works are related to biosensors [13–17]. Since its first use for modified electrodes [18], zeolite has generated great interest, finding numerous applications in electroanalysis [19,20]. The advantages of zeolite compared to other modifications are unique size, shape and charge se- lectivity, ion exchange and catalytic capacity. Walcarius [21] and Rolison [22] reviewed the utilization of the zeolite in the modification of electrode. The voltammetric sensors based on zeolite modified electrode have been developed for deter- mination of dopamine [23], glucose [24], phenols [25] and herbicides [26]. Recently, Doménech et al. [27] described the electrochemical behavior of 2,4,6-triphenylpyrylium ion incorporated in zeolite Y modified electrode for catalytic 0039-9140/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2004.01.018