Internal standard solution— Dissolve cholestane in chloroform to obtain a solution having a concentration of about 0.5mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Miconazole Nitrate RSin a mixture of chloroform and methanol (1:1)to obtain a solution having a known concentration of about 0.8mg per mL.Transfer 5.0mLof this solution to a test tube,add 2.0mLof Internal standard solution,and evaporate at a temperature not higher than 40with the aid of a current of nitrogen to dryness.Dissolve the residue in 2.0mLof a mixture of chloroform and methanol (1:1),and mix to obtain a Standard preparationhaving a known miconazole nitrate concentration of about 2mg per mL.

Assay preparation— Transfer an accurately weighed portion of Topical Powder,equivalent to about 20mg of miconazole nitrate,to a stoppered 50-mLcentrifuge tube.Add 25.0mLof methanol,and shake by mechanical means for 30minutes to dissolve the miconazole nitrate.Centrifuge to obtain a clear supernatant.Transfer 5.0mLof this solution to a test tube,add 2.0mLof Internal standard solution,and evaporate at a temperature not higher than 40with the aid of a current of nitrogen to dryness.Dissolve the residue in 2.0mLof a mixture of chloroform and methanol (1:1).

Chromatographic system (see Chromatography á621ñ)—The gas chromatograph is equipped with a flame-ionization detector and a 1.2-m ×2-mm glass column containing 3percent phase G32on support S1A.The injection port,detector,and column are maintained at temperatures of about 250,300,and 250,respectively.Helium is used as the carrier gas,at a flow rate of about 50mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed under Procedure:the resolution,R,between the cholestane and miconazole nitrate peaks is not less than 2.0,and the relative standard deviation for replicate injections is not more than 3.0%.

Procedure— Separately inject equal volumes (about 5µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times for cholestane and miconazole nitrate are about 0.5and 1.0,respectively.Calculate the quantity,in mg,of miconazole nitrate (C18H14Cl4N2O·HNO3)in the portion of Topical Powder taken by the formula: in which Cis the concentration,in mg per mL,of USP Miconazole Nitrate RSin the Standard preparation,and RUand RSare the peak response ratios of the miconazole nitrate peak to the cholestane peak obtained from the Assay preparationand the Standard preparation,respectively.