Spectrophotometric determination of melatonin and pyridoxine HCI in binary mixture using first derivative of the ratio spectra method

Ratio spectra of the first derivative spectrophotometric method for determination of melatonin (MEL) and pyridoxine HCl (PYR) in tablets in the presence of each other has been described. This technique depends on first derivative of the ratio spectra by division of the absorption spectrum of the binary mixture by a standard spectrum of one of the components and then calculating the first derivative of the ratio spectrum. The first derivative of the ratio amplitudes at 218.3 and 232.7 nm for MEL and 212.3, 282.6, and 304.6 nm for PYR were selected for the determination. The calibration graphs follow Beer's law in the ranges of 1-30 mug mL(-1) for MEL and 1-22 mug mL(-1) for PYR. The proposed method was successfully applied to the determination of these compounds in synthetic mixtures and in pharmaceutical preparations. The proposed method is simple, rapid and suitable for quality control applications.