Lipoic acid is an important participant of the process of biological oxidation in both lipid and carbohydrate metabolism, produces a lipotropic effect by reducing the blood cholesterol level, and exhibits a pronounced antidote action in cases of poisoning with mercury, arsenic, and cadmium. Lipoic acid is administered for the treatment of atherosclerosis, ischemic heart disease, liver disorders, and various intoxicated states. This compound enters both into monopreparations (tabletized or capsulated) and in complex mixtures with vitamins and trace elements (e.g., in complevit, tivit, and saltivit additives).Analysis of lipoic acid encounters certain difficulties because this compound is susceptible to various transformations, primarily via oxidation and polymerization. The use of conventional methods such as spectrophotometry or polarography for the determination of lipoic acid in medicinal preparations shows low precision because of low extinction (in spectrophotometry) and high sensitivity to impurities with close redox potentials (in polarography), these factors hindering the correct analysis.HPLC is widely used and proved to be a universal method for the determination of lipoic acid and vitamins (A, E, etc.) in multivitamin complexes, making possible simultaneous analysis for these components in the same sample. We have checked this possibility both on lipoic acid monopreparations (tablets) and on new multivitamin complexes (tivit, saltivit) containing water-and fat-soluble vitamins, lipoic and succinic acid, and trace elements.An important advantage of the proposed HPLC technique is the simple sample preparation procedure, which significantly reduces the total duration of analysis and ensures simultaneous determination of the content of lipoic acid and vitamins A and E in these drugs.
EXPERIMENTAL PARTThe samples were prepared using methanol, acetonitrile, and isopropanol of chromatographic purity grade, rectified ethyl alcohol, trifluoroacetic acid (TFA), and doubly distilled water.Preparation of sample solutions. Multivitamin pills to be tested are assumed to contain 0.002 -0.030 g of lipoic acid, 800 -4300 IU of retinol acetate (vitamin A), and 0.005 -0.030 g of a-tocopherol acetate (vitamin E). To several (2 -5) tablets in a 50-ml flat-bottom flask was added 7.5 ml of 0.05 M aqueous HCl and the mixture was kept with continuous stirring on a water bath at 60 ± 5°C until complete disintegration of the sample preparation, after which the flask was cooled in a stream of cold water. To this mixture was added 5 ml of ethyl ether and the whole was stirred for 2 -3 min. Then, 10 -15 ml of 96% ethanol was added and the mixture was stirred and quantitatively transferred to a 50-ml measuring flask with a glass-ground stopper. The flask was filled with the same solvent to the mark, after which the contents was thoroughly stirred and treated in a centrifuge. Immediately prior to analysis, the solution was filtered through a Millipore filter with a pore size of 0.2 -0.4 mm.Reference retinol acetate solution. An exactl...