A: Infrared Absorption 197K—If a difference appears, dissolve portions of both the test specimen and the Reference Standard in methanol, evaporate the solutions to dryness, and repeat the tests.

B: Dissolve about 6 mg in 2 mL of sulfuric acid, and allow to stand for 5 minutes: an orange color is produced. Pour the solution into 10 mL of water: the color changes first to yellow and then, gradually, to bluish green.

Test solution: 5 mg per mL, in dioxane.

Mobile phase— Prepare a filtered and degassed mixture of chloroform and methanol (98:2). Make adjustments if necessary (see System Suitability under Chromatography 621).

Test solution— Transfer about 25 mg of Prednisone, accurately weighed, to a suitable container, dissolve in 20 mL of Mobile phase, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 6.0-mm × 4.0-cm column that contains packing L3. The flow rate is about 1 mL per minute. Chromatograph the Test solution, and record the peak responses as directed for Procedure: the column efficiency is not less than 2,500 theoretical plates; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Inject a volume (about 5 µL) of the Test solution into the chromatograph, record the chromatogram, and measure the peak responses. Calculate the percentage of each impurity in the portion of Prednisone taken by the formula: in which ri is the peak response for each impurity; and rs is the sum of the responses of all peaks: not more than 1.5% of any individual impurity is found, and not more than 2.0% of total impurities is found.

Mobile phase— Prepare a suitable filtered mixture of water, peroxide-free tetrahydrofuran, and methanol (688:250:62) such that at a flow rate of 1.0 mL per minute, the retention times of prednisone and acetanilide are about 8 and 6 minutes, respectively.

Internal standard solution— Prepare a solution of acetanilide in dilute methanol (1 in 2) having a concentration of about 110 µg per mL.

Standard preparation— Using an accurately weighed quantity of USP Prednisone RS, prepare a solution in dilute methanol (1 in 2) having a known concentration of about 0.2 mg per mL. Transfer 5.0 mL of this solution and 5.0 mL of the Internal standard solution to a 50-mL volumetric flask. Add dilute methanol (1 in 2) to volume, and mix to obtain a Standard preparation having a known concentration of about 20 µg of USP Prednisone RS per mL. Prepare this solution fresh.

Assay preparation— Using about 50 mg of Prednisone, accurately weighed, proceed as directed for Standard preparation, beginning with “prepare a solution in dilute methanol (1 in 2).”

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 25-cm column that contains packing L1. Chromatograph five replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 2.0%; and the resolution factor between prednisone and the internal standard is not less than 3. Adjust the operating parameters so that the peak obtained from the Standard preparation is about one-half full-scale.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph by means of a suitable microsyringe or sampling valve, record the chromatograms, and measure the responses at equivalent retention times. Calculate the quantity, in mg, of C21H26O5 in the portion of Prednisone taken by the formula: in which C is the concentration, in µg per mL, of USP Prednisone RS in the Standard preparation; and RU and RS are the peak response ratios of the prednisone peak to the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.