Melatonin (N-acetyl-3-(2-aminoethyl)-5-methoxyindole) is biologically active as a neurohormone and antioxidant agent. The optimized dispersive liquid-liquid microextraction (DLLME) followed by high-performance liquid chromatography-ultra violet detection (HPLC-UV) was used for the analysis of melatonin in human plasma. Influence variables such as volume of extracting (carbon tetrachloride: CCl4) and dispersing solvents (acetonitrile: ACN), pH and ionic strength, extraction time and centrifugation time were screened in a 2(6-2) fractional factorial design (FFD) and then the significant variables were optimized by using a central composite design (CCD). At optimum conditions values of variables set as pH 6.0, 1.5 mL ACN, 140 μL CCl4, 1.0 min extraction time and 3.0 min centrifugation at 4,500 rpm. At optimum conditions method has linear response over 2.0-500.0 ng mL(-1) with detection limit of 0.5 ng mL(-1) with relative standard deviations (RSDs) less than 5.0%. The values of intra-day and inter-day RSD were 4.3% and 8.5%, respectively. The method was applied successfully for the analysis of melatonin in plasma sample.
