Dilute phosphoric acid— Dilute 10mLof phosphoric acid with water to 100mL.

Phosphate buffer— Dissolve 1.38g of monobasic sodium phosphate in 1000mLof water,adjust with Dilute phosphoric acidto a pHof 3.0,and mix.

Diluting solution— Prepare a mixture of water and Dilute phosphoric acid(9:1).

Mobile phase— [NOTE—Phosphate buffer and tetrahydrofuran may be filtered and degassed separately beforemixing.]Prepare a filtered and degassed mixture of Phosphate bufferand tetrahydrofuran (58:42).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Tretinoin RSin tetrahydrofuran to obtain a solution having a known concentration of about 0.4mg per mL.Dilute a known volume of this solution,quantitatively and stepwise if necessary,with a mixture of tetrahydrofuran and Diluting solution(3:2)to obtain a solution having a known concentration of about 4µg per mL.

Assay preparation— Transfer an accurately weighed quantity of Cream,equivalent to 1.0mg of tretinoin,to a 50-mLvolumetric flask,and add 20.0mLof tetrahydrofuran.Shake the flask to disperse the cream,dilute with tetrahydrofuran to volume,mix,and filter,if necessary.Transfer 5.0mLof this solution to a 25-mLvolumetric flask,dilute with a mixture of tetrahydrofuran and Diluting solution(3:2)to volume,mix,and filter.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 365-nm detector and a 3.9-mm ×15-cm column that contains 4-µm packing L1.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 25µL)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 tretinoin (C20H28O2)in the portion of Cream taken by the formula: in which Cis the concentration,in µg per mL,of USP Tretinoin RSin the Standard preparation,and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.