RESUMO
Melatonin can be sensitively detected in pharmaceuticals by two different and simple electrochemical methods: cyclic voltammetry (CV) and amperometric detection in a flow injection analysis system (FIA-ED). An adequate pre-treatment of the carbon paste electrode in the first case and the employ of a high flow rate in the second one were the key for obtaining a very good reproducibility (R.S.D. values of 1.5 (n=10) and 1.3% (n=20), respectively). Low limits of detection were achieved and with the coupling of a flow system a linear dynamic range of three orders of magnitude (from 10(-8) to 10(-5) M) was obtained. Both methods were applied to the determination of melatonin in pharmaceuticals. In order to best validate these methodologies a fluorescent procedure was developed to contrast the results. As no interferences from the matrix were found the employ of a separation technique is not necessary. In this way the procedure is fastened and simplified. Moreover, the low price, ease of handling, possibility of automation and high sample throughput are important advantages that convert the flow methodology in an attractive alternative for quality control of pharmaceuticals.