Comparison of analytical methods for the quality control of a new formulation containing soy extract and melatonin

Three analytical methods have been developed and compared for the quality control of a new formulation (Soymen GN® capsules) containing soy extract and melatonin for the treatment of menopausal symptoms. The first method is based on micellar electrokinetic chromatography (MEKC) with diode-array detection, using a mixture of basic carbonate buffer (95%) and methanol (5%), containing 55 mM SDS, as the background electrolyte. The second method is a high-performance liquid chromatographic (HPLC) method with ultraviolet detection at 260 nm. The third method is an HPLC method coupled to amperometric detection which is carried out at an oxidation potential of + 0.8 V. In both HPLC systems, the chromatographic separation is obtained on a reversed-phase C18 column using a mixture of acetonitrile and an acidic phosphate buffer (25/75, v/v) as the mobile phase. A feasible pre-treatment procedure with a methanol/water mixture has been implemented to achieve the quantitative extraction of the main soy isoflavones and of melatonin from the capsules. The results obtained with the three methods are in good agreement with each other and satisfactory in terms of linearity (r2 > 0.9996), precision (RSD < 5.4%) and accuracy (recovery > 97%). Thus, each of the three analytical methods seems to be suitable for the simultaneous analysis of the main soy isoflavones and melatonin in the new commercial formulation.