Colorimetric and spectrofluorimetric methods for the determination of melatonin in tablets and serum

Two sensitive and accurate colorimetric and spectrofluorimetric methods are presented for the determination of melatonin in tablets and serum. The first method utilizes the reactions of p-dimethylaminobenzaldehyde in hydrochloric acid (van Urk reagent)-ferric chloride in sulphuric acid(Salkowski reagent)mixture. The blue color of the resulting reaction product Is measured at 630 The second method is based on the reaction of melatonin with o-phthalaldehyde in acid medium which yields highly fluorescent condensation product that is measured at 465 nm as emission wavelength, using excitation wavelength at 355 run. No interference was observed from tableting additives, and the applicability of the methods was examined by analysing tablets containing melatonin(single and combined with pyridoxine). Mean percentage recoveries from tablets were found to be 99.9+/-0.31 for single and 100.5+/-0.15 for combined tablets using colorimetric method, while by applying spectrofluorimetric method the recoveries were found to be 100.6+/-0.41 for single and 100.2+/-0.39 for combined tablets. Furthermore. the proposed methods were extended to the in-vitro determination of melatonin in serum. The detection limits are 0.27 ug ml(-1) for colorimetric method and 0.00035 ug ml(-1) for spectrofluorimetric method.