Mobile phase— Prepare a mixture of n-hexane and dehydrated alcohol (4000:1).Make adjustments,if necessary (see System Suitabilityunder Chromatography á621ñ).

Reference solution 1— Dissolve an accurately weighed quantity of USP Probucol Related Compound A RSin n-hexane,and dilute quantitatively and stepwise with n-hexane to obtain a solution having a known concentration of about 10µg per mL.

Reference solution 2— Dissolve an accurately weighed quantity of USP Probucol Related Compound B RSin n-hexane,and dilute quantitatively with n-hexane to obtain a solution having a known concentration of about 0.1mg per mL.

Reference solution 3— Dissolve an accurately weighed quantity of USP Probucol Related Compound C RSin n-hexane,and dilute quantitatively with n-hexane to obtain a solution having a known concentration of about 1mg per mL.

Standard solution— Transfer about 10mg of USP Probucol RS,accurately weighed,to a 50-mLvolumetric flask.Add 1mLof Reference solution 1,4mLof Reference solution 2,and 10mLof Reference solution 3,and dilute with n-hexane to volume.

Test solution— Transfer about 1g of Probucol,accurately weighed,to a 25-mLvolumetric flask,dissolve in and dilute with n-hexane to volume,and mix.

System suitability solution— Pipet 1mLof Reference solution 2and 1mLof the Test solutioninto a 200-mLvolumetric flask,dilute with n-hexane to volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with 254-nm and 420-nm detectors,connected in series,and a 4.6-mm ×25-cm column that contains packing L3.The flow rate is about 1mLper minute.Chromatograph the System suitability solution,and record the peak responses detected at 254nm,for related compound Band probucol.Related compound Belutes first;the resolution,R,of the peaks is not less than 2.5;and the relative standard deviation for replicate injections,determined from the probucol peak,is not more than 2%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the peak areas.The order of elution is compound C,compound B,compound A,and finally probucol.Compound Ais detected at 420nm,and the others are detected at 254nm.Calculate the percentage of each related compound in the portion of Probucol taken by the formula: in which Cis the concentration,in mg per mL,of the respective USP Probucol Related Compound RSin the Standard solution;Wis the weight,in mg,of Probucol taken;and rUand rSare the peak areas obtained from the Test solution;and the Standard solution,respectively:not more than 0.0005%of compound A;not more than 0.02%of compound B;and not more than 0.5%of compound Care found.

Solvent— Use dimethyl sulfoxide.

Mobile phase— Prepare a degassed and filtered mixture of acetonitrile and water (85:15).Make adjustments,if necessary (see System Suitabilityunder Chromatography á621ñ).

System suitability preparation— To about 56mg of Probucol add 10mLof n-propyl alcohol,and dissolve the Probucol.Add 1.0mLof 70%tert-butyl hydroperoxide,and mix.Cover loosely,and heat on a steam bath at about 90for 30minutes.Allow to cool to room temperature,dilute with a mixture of n-propyl alcohol and water (17:14)to 200mL,and mix.Dilute 25mLof this solution with Mobile phaseto 100mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Probucol RSin Mobile phase,and dilute quantitatively and stepwise with Mobile phaseto obtain a solution having a known concentration of about 63µg per mL.

Assay preparation— Transfer about 63mg of Probucol,accurately weighed,to a 50-mLvolumetric flask.Dissolve in and dilute with Mobile phaseto volume,and mix.Pipet 5mLof this solution into a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 242-nm detector and a 4.6-mm ×25-cm column that contains packing L7.The flow rate is about 2.0mLper minute.Chromatograph the System suitability preparation,and record the peaks for the degradation product and probucol at relative retention times of approximately 0.8and 1.0,respectively.The resolution,R,of the peaks is not less than 2.0.Chromatograph the Standard preparation:the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 50µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the areas for the major peaks.Calculate the quantity,in mg,of C31H48O2S2in the portion of Probucol taken by the formula: in which Cis the concentration,in µg per mL,of USP Probucol RSin the Standard preparation;and rUand rSare the peak areas obtained from the Assay preparationand the Standard preparation,respectively.