Mobile phase— Mix approximately equal volumes of acetonitrile and water, adjusting the ratio of solvents as necessary to achieve acceptable chromatography.

Internal standard solution— Dissolve Butylparaben in acetonitrile to obtain a solution having a concentration of 6 µg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Halcinonide RS in Internal standard solution to obtain a solution having a known concentration of about 0.04 mg per mL. Mix 5.0 mL of this solution with 5.0 mL of the Mobile phase. Each mL of the Standard preparation has a known concentration of about 0.02 mg of USP Halcinonide RS.

Assay preparation— Transfer an accurately weighed quantity of Ointment, equivalent to about 1 mg of halcinonide, to a glass-stoppered, 50-mL centrifuge tube, and add 25.0 mL of Internal standard solution and 5.0 mL of hexane. Place in a water bath at 58 ± 2 for 3 minutes, then mix in a vortex mixer for about 1 minute until the specimen is well dispersed. Repeat the above-specified heating and mixing step one more time. Cool in an ice-methanol bath for 15 minutes or until the two phases separate, centrifuging if necessary. Transfer 5.0 mL of the lower layer into a 15-mL centrifuge tube, add 5.0 mL of Mobile phase, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: 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 3.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.6 for butylparaben and 1.0 for halcinonide. Calculate the quantity, in mg, of C24H32ClFO5 in the portion of Ointment taken by the formula: in which C is the concentration, in mg per mL, of USP Halcinonide RS in the Standard preparation; and RU and RS are the ratios of the peak responses of halcinonide to internal standard obtained from the Assay preparation and the Standard preparation, respectively.