Improved extraction methods for avoiding the interference of copper in the LC determination of ascorbic acid in multivitamin-mineral tablets.

A rapid sample extraction procedure for the determination of ascorbic acid (AA) by high performance liquid chromatography (HPLC) in multivitamin-mineral formulations containing interfering copper has been developed. The method takes special precautions to prevent degradation of AA in contact with high concentrations of interfering elements such as copper. Sample preparation involved addition of pyrogallol, citric acid solution and short time extraction (5 min) under an atmosphere of nitrogen to prevent oxidation of AA. Another sample cleanup based on extraction of the multivitamin-mineral tablet with the extraction solution as described above, but with addition of strong cation exchange sorbent, was also developed. The copper and other minerals in the formula were retained on an ion exchange sorbent and thus represses the speed of oxidation of AA. Extracts of multivitamin-mineral tablets were analyzed by ion-pair reversed phase HPLC and UV-diode array detection. Ten multivitamin formulations containing copper were analyzed by the two proposed methods. The analytical results of AA obtained using the simple sample extraction method and the method involving addition of cation exchange were 89-115% and 95-112%, respectively, of declared concentrations. The parameters for the validation of the methods are given.