(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of methanol and 0.05 M monobasic potassium phosphate (350:300). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Dissolve a quantity of betamethasone in methanol to obtain a solution containing about 0.4 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Prednisone RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.25 mg per mL. Transfer 10.0 mL of this solution to a 50-mL volumetric flask. Add 10.0 mL of Internal standard solution, dilute with methanol to volume, and mix.

Assay preparation— Transfer an accurately measured volume of well-mixed Suspension, equivalent to about 80 mg of prednisone, to a 100-mL volumetric flask. Dilute with methanol to volume, and mix. Transfer 3.0 mL of this solution to a 50-mL volumetric flask. Add 10.0 mL of Internal standard solution, dilute with methanol to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-cm column that contains packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 1.9 for betamethasone and 1.0 for prednisone, the resolution, R, between the prednisone peak and the betamethasone peak is not less than 3.5, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the analyte peaks. Calculate the quantity, in mg, of prednisone in each mL of the Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Prednisone RS in the Standard preparation, V is the volume, in mL, of the Suspension taken to prepare the Assay preparation, and RU and RS are the ratios of the prednisone peak to the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.