Flow-injection amperometric determination of dopamine in pharmaceuticals using a polyphenol oxidase biosensor...

The amperometric determination of dopamine (Do) in pharmaceuticals formulations by flow injection analysis (FIA) is proposed. An enzymatically modified carbon paste electrode constituted by 25% (w/w) of polyphenol oxidase obtained from Annona muricata L. tissue, 30% (w/w) of graphite, 30% (w/w) of silicone and 15% (w/w) of 7,7,8,8 tetracyanoquinodimethane (TCNQ), was used as flow-through detector. The flow amperometric detection was carried out at a potential of 0.10 V (vs. Ag/AgCl) when an injected sample volume of 250 microl was inserted on a 0.3 M phosphate buffer carrier solution (pH 7.8) flowing at 2.5 ml/min. The developed biosensor showed good stability and reproducibility, enabling up to 500 determinations in 60 days, without considerable loss of enzymatic activity. The FIA system presented a linear response to Do concentrations in the interval from 2 x 10(-2) to 2 x 10(-4) M, with relative standard deviations lower than 1.5%. The kinetic parameter K(M) for the soluble and immobilized enzyme was 1.45 x 10(-2) and 1.91 x 10(-2) M, respectively. In the analyses of different commercially pharmaceutical formulations a relative deviation lower than about 3.4% was obtained.