The average weight per capsule or tablet was calculated from the weight of 10 capsules or tablets. Five capsules or tablets were finely ground and homogenized in an agate mortar. The amount of the powdered mass analyte corresponding to one capsule or tablet
was dissolved in the mobile phase, and the solution was diluted appropriately so that the concentration of -lipoic acid in the final test solution was within the linear dynamic range (0.01–60g/mL).The solution was filtered through a 0.45-mnylon membrane filter
before injection into the HPLC-electrochemical system.