Mobile phase— Prepare a filtered and degassed mixture of methanol,water,and glacial acetic acid (65:35:1).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Internal standard solution— Prepare a solution of ethylparaben in methanol having a concentration of about 0.04mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Desoximetasone RSin methanol to obtain a solution having a known concentration of about 0.4mg per mL.Pipet 5mLof this solution into a 50-mLcentrifuge tube.Add 10.0mLof Internal standard solution,dilute with methanol quantitatively to 40.0mL,and mix to obtain the Standard preparationhaving a known concentration of about 0.05mg per mL.

Assay preparation— Transfer an accurately weighed amount of Cream,equivalent to about 2mg of desoximetasone,to a 50-mLcentrifuge tube,and add a few 3-mm glass beads.Add 10.0mLof Internal standard solutionand about 30mLof methanol,and mix.Tightly cap the centrifuge tube,and immerse it for 10minutes in a bath maintained at a temperature of 65.Remove the tube from the bath,and immediately vortex at high speed for 30seconds.Return the tube to the hot water bath for 5minutes,remove it from the bath,and immediately vortex for 30seconds.Repeat the procedure one more time,then cool the tube in an ice-bath held at 10until no further flocculent precipitation occurs.Centrifuge,and use the supernatant.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm ×15-cm column that contains packing L7.The flow rate is about 1mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 1and 2for ethylparaben and desoximetasone,respectively;the tailing factor for the analyte peak is not more than 2.0;the resolution,R,between the analyte and internal standard peaks is not less than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.

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 C22H29FO4in the portion of Cream taken by the formula: in which Cis the concentration,in mg per mL,of USP Desoximetasone RSin the Standard preparation;and RUand RSare the peak response ratios obtained from the Assay preparationand the Standard preparation,respectively.