Medium: water; 900 mL.

Apparatus 1 (for hard gelatin capsules): 100 rpm.

Apparatus 2 (for soft gelatin capsules): 75 rpm.

Time: 60 minutes.

Mobile phase and Chromatographic system— Proceed as directed in the test for Content of alpha lipoic acid.

Standard solution— Dissolve an accurately weighed quantity of USP Alpha Lipoic Acid RS in a mixture of acetonitrile and water (1:1) to obtain a solution having a known concentration of 1 mg per mL. Transfer 1 mL of this solution to a 50-mL volumetric flask, dilute with water to volume, and mix to obtain a solution having a known concentration of about 0.02 mg per mL.

Test solution— Withdraw a portion of the solution under test, and filter, discarding the first portion of the filtrate. Transfer an accurately measured aliquot to a volumetric flask, and dilute with water to volume to obtain a solution having an expected concentration of about 0.02 mg of alpha lipoic acid per mL.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the peak areas. Determine the amount of C8H14O2S2 dissolved by the formula: in which C is the concentration of USP Alpha Lipoic Acid RS in the Standard solution; D is the dilution factor of the Test solution; and rU and rS are the peak areas of alpha lipoic acid obtained from the Test solution and the Standard solution, respectively.

Tolerances— Not less than 70% of the labeled amount of C8H14O2S2 is dissolved in 60 minutes.

Mobile phase— Prepare a filtered and degassed mixture of 0.025 M phosphoric acid and acetonitrile (62:38).

Standard solution— Dissolve an accurately weighed quantity of USP Alpha Lipoic Acid RS in acetonitrile, and dilute quantitatively, and stepwise if necessary, with acetonitrile to obtain a solution having a known concentration of 0.05 mg per mL.

Test solution 1 (for hard gelatin capsules)— Empty and mix thoroughly the contents of not fewer than 20 Capsules. Transfer an accurately weighed portion of the powder, equivalent to about 100 mg of alpha lipoic acid, to a suitable container, add about 70 mL of a mixture of acetonitrile and water (1:1), and shake for 45 minutes by mechanical means. Transfer to a 100-mL volumetric flask, dilute with the mixture of acetonitrile and water (1:1) to volume, mix, and filter a portion of this preparation, discarding the first 5 mL of the filtrate. Transfer 5.0 mL of the remaining filtrate to a 100-mL volumetric flask, dilute with water to volume, and mix.

Test solution 2 (for soft gelatin capsules)— Using a suitable cutting instrument, open an accurately counted number of Capsules equivalent to about 500 mg of alpha lipoic acid. Transfer the contents and the shells to a suitable container with stopper, add 500.0 mL of a mixture of acetonitrile and water (1:1), and shake for 45 minutes by mechanical means. Filter a portion of this preparation, discarding the first 5 mL of the filtrate. Transfer 5.0 mL of the remaining filtrate to a 100-mL volumetric flask, dilute with water to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 220-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor for alpha lipoic acid is not more than 1.2; the efficiency of the column is not less than 1300 theoretical plates; and the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the appropriate Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of alpha lipoic acid in the portion of hard gel capsules taken by the formula: in which C is the concentration, in mg per mL, of USP Alpha Lipoic Acid RS in the Standard solution; and rU and rS are the peak responses of alpha lipoic acid obtained from Test solution 1 and the Standard solution, respectively. Calculate the quantity, in mg, of alpha lipoic acid in each soft gel capsule taken by the formula: in which C is the concentration, in mg per mL, of USP Alpha Lipoic Acid RS in the Standard solution; N is the number of capsules taken; and rU and rS are the peak responses of alpha lipoic acid obtained from Test solution 2 and the Standard solution, respectively.

(Official January 1, 2007)