Identification—
Transfer an amount of Gel, equivalent to 100 µg of desoximetasone, to a 15-mL centrifuge tube. Add 3 mL of acetonitrile, sonicate for approximately 1 minute, centrifuge, and transfer the clear supernatant to another 15-mL centrifuge tube. Evaporate the solution under nitrogen at a temperature between 35
and 45
to dryness. Dissolve the residue in 100 µL of methanol, using a sonicator. Streak separately the entire test solution and 100 µL of a Standard solution of
USP Desoximetasone RS in methanol containing 1 mg per mL on a thin-layer chromatographic plate (see
Chromatography 
621
) coated with a 0.25-mm layer of chromatographic silica gel mixture and an area of preadsorbent material on which specimens are applied. Allow the streaks to dry, and develop the chromatogram in a saturated chamber containing a mixture of acetone and chloroform (1:1). Allow the solvent front to move not less than 10 cm beyond the origin. After drying, examine the plate under UV light at 254 nm: the
RF value of the principal spot obtained in the chromatogram of the test solution corresponds to that of the Standard solution.
Alcohol content—
Transfer about 2.5 g of Gel, accurately weighed, to a 50-mL volumetric flask. Dissolve in methanol, dilute with methanol to volume, and mix. Determine the alcohol content of the specimen thus prepared by the
Method II—Gas-Liquid Chromatographic Method (see
Alcohol Determination 611), using isopropyl alcohol as the internal standard and using methanol in place of water as the solvent: between 18.0% and 24.0% (w/w) of C
2H
5OH is found.
Assay—
Mobile phase—
Prepare a suitable filtered and degassed solution of methanol, water, and glacial acetic acid (65:35:1). Adjust the ratio, if necessary, so that the retention time of desoximetasone is about 8 minutes.
Standard preparation—
Using an accurately weighed quantity of
USP Desoximetasone RS, prepare a solution in methanol containing 0.5 mg per mL. Dilute an accurately measured volume of this solution with methanolic calcium chloride dihydrate solution (1.5 in 100) to obtain a
Standard preparation having a known concentration of about 0.025 mg per mL.
Assay preparation—
Transfer an accurately weighed quantity of Gel, equivalent to about 1.25 mg of desoximetasone, to a 50-mL volumetric flask, add approximately 40 mL of methanolic calcium chloride dihydrate solution (1.5 in 100), and sonicate to disperse the gel. Dilute with the same solution to volume, mix, and centrifuge. Use the clear supernatant.
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 1.5 mL per minute. 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%.
Procedure—
Separately inject equal volumes (about 10 µL) of the
Standard preparation and the
Assay preparation into the chromatograph by means of a sampling valve, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C
22H
29FO
4 in the portion of Gel taken by the formula:
50C(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Desoximetasone RS in the
Standard preparation; and
rU and
rS are the peak responses of desoximetasone obtained from the
Assay preparation and the
Standard preparation, respectively.
