Test solution— Transfer 2.5g of Alpha Lipoic Acid,accurately weighed,to a 50-mLvolumetric flask,dissolve in 30mLof dehydrated alcohol,and dilute with dehydrated alcohol to volume.

PS(CS/CU)(rU/rS),

Adsorbent: 0.25-mm layer of chromatographic silica gel mixture.

Test solution— Dissolve an accurately weighed quantity of Alpha Lipoic Acid in dimethylformamide to obtain a solution having a concentration of about 40.0mg per mL.

Standard solution 1— Dissolve an accurately weighed quantity of USP Alpha Lipoic Acid RSin dimethylformamide to obtain a solution having a known concentration of about 40.0mg per mL.This solution contains 2.0%of polymer.Store in low-actinic glassware.

Standard solution 2— Dilute an aliquot of Standard solution 1with a sufficient amount of dimethylformamide to obtain a solution having a known concentration of about 20.0mg per mL.This solution contains 1.0%of polymer.

Standard solution 3— Dilute an aliquot of Standard solution 2with a sufficient amount of dimethylformamide to obtain a solution having a known concentration of about 10.0mg per mL.This solution contains 0.5%of polymer.

Application volume: 5µLof each solution.

Developing solvent system: a mixture of n-propyl alcohol,ethyl acetate,water,and 25percent ammonia water (40:40:10:5).Allow the chamber to become saturated for at least 1hour.

Iodine vapor saturated chamber— Transfer 4.0g of iodine crystals,accurately weighed,to a small watch glass,and place into the chromatography chamber.Allow the chamber to become saturated for at least 2hours.

Procedure— Proceed as directed for Thin-Layer Chromatographyunder Chromatography á621ñ,except to develop the chromatogram until the solvent front has moved 10cm.Remove the plate,and allow to air-dry until the ammonia disappears completely.Heat at 50for 20minutes,cool the plate,and place in the Iodine vapor saturated chamberuntil the spots are visible.The chromatograms exhibit a spot due to alpha lipoic acid polymer at an RFvalue of 0.0and a spot due to alpha lipoic acid at an RFvalue between 0.25and 0.30.The spot due to polymeric alpha lipoic acid in the chromatogram obtained from the Test solutionis not more intense than the spot in the chromatogram obtained from Standard solution 1:not more than 2%is found.

0.005M Phosphate solution— Dissolve 1.36g of monobasic potassium phosphate in 2000mLof water.

Phosphoric acid solution— Transfer 8.3mLof phosphoric acid to a 100-mLvolumetric flask,and dilute with water to volume.

Mobile phase— Prepare a suitable filtered and degassed mixture of methanol,0.005M Phosphate solution,and acetonitrile (1160:920:180).Adjust with Phosphoric acid solutionto a pHof 3.0to 3.1.

Solvent buffer— Prepare a suitable filtered and degassed mixture of 0.005M Phosphate solutionand acetonitrile (1:1).Adjust with Phosphoric acid solution to a pHof 3.5to 3.7.

Standard preparation— Dissolve an accurately weighed quantity of USP Alpha Lipoic Acid RSin Solvent bufferto obtain a solution having a known concentration of about 1.0mg per mL.

Assay preparation— Dissolve an accurately weighed quantity of Alpha Lipoic Acid in Solvent bufferto obtain a solution having a concentration of about 1.0mg per mL.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 215-nm detector and a 4.6-mm ×250-mm column that contains packing L1.The flow rate is about 1.2mLper minute.The column temperature is maintained at 35.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the retention times for alpha lipoic acid and 6,8-epitrithiooctanoic acid are about 6.5minutes and 13minutes,respectively;the column efficiency is not less than 15,000theoretical plates;the tailing factor for the alpha lipoic acid peak is not more than 2;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the areas for the major peaks.Calculate the percentage of C8H14O2S2in the portion of Alpha Lipoic Acid taken by the formula: in which PSis the percentage of alpha lipoic acid in USP Alpha Lipoic Acid RS;CSis the concentration,in mg per mL,of USP Alpha Lipoic Acid RSin the Standard preparation;CUis the concentration of USP Alpha Lipoic Acid RSin the Assay preparation;and rUand rSare the peak areas for alpha lipoic acid obtained from the Assay preparationand the Standard preparation,respectively.