Solution A— Prepare a filtered and degassed solution in water containing 17g of tetrabutylammonium hydrogen sulfate per liter.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Solution B— Use filtered and degassed acetonitrile.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Mobile phase— Use variable mixtures of Solution Aand Solution Bas directed for Chromatographic system.

Standard solution— Dissolve an accurately weighed quantity of USP Astemizole RSin methanol,and dilute quantitatively,and stepwise if necessary,with methanol to obtain a solution having a known concentration of about 25µg per mL.

Resolution solution— Dissolve an accurately weighed quantity of USP Astemizole RSand ketoconazole in methanol,and dilute quantitatively,and stepwise if necessary,with methanol to obtain a solution having a known concentration of about 25µg and 250µg per mL,respectively.

Test solution— Transfer about 100mg of Astemizole,accurately weighed,to a 10-mLvolumetric flask,dissolve in and dilute with methanol to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 278-nm detector and a 4.6-mm ×10-cm column that contains base-deactivated 3-µm packing L1.The flow rate is about 1mLper minute.Equilibrate the system with acetonitrile and then with 95%Solution Aand 5%Solution B,and hold at that composition for 5minutes prior to injection.After injection,linearly change the composition to 80%Solution Aand 20%Solution Bover a period of 15minutes.Maintain this composition for an additional 3minutes.Purge the column with 100%Solution Bfor 5minutes,and then equilibrate the system to the initial composition for 5minutes prior to the following injection.Chromatograph the Resolution solution,and record the peak responses as directed for Procedure:the resolution,R,between the astemizole and ketoconazole peaks is not less than 1.5.

Procedure— Separately inject equal volumes (about 10µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the peak responses.Calculate the percentage of each impurity in the portion of Astemizole taken by the formula: in which riis the peak response for each impurity,and rSis the peak response of the Standard solution:not more than 0.25%of any individual impurity is found,and not more than 0.5%of total impurities is found.

Mobile phase— Prepare a filtered and degassed mixture of methanol,0.13Mammonium acetate,acetonitrile,and diethylamine (470:300:230:1.0),and adjust with glacial acetic acid to a pHof 7.5.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Astemizole RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 1.0mg per mL.

Assay preparation— Transfer about 50mg of Astemizole,accurately weighed,to a 50-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 220-nm detector and a 4.6-mm ×25-cm column that contains packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the column efficiency is not less than 4000theoretical plates;the tailing factor is not more than 1.8;and the relative standard deviation for replicate injections is not more than 1.5%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C28H31FN4Oin the portion of Astemizole taken by the formula: in which Cis the concentration,in mg per mL,of USP Astemizole RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.