Identification—
Evaporate 25 mL of the
Assay preparation, prepared as directed in the
Assay, on a steam bath just to dryness, and dissolve the residue in 2 mL of acetonitrile. This is the test solution. Prepare a Standard solution of
USP Desoximetasone RS in acetonitrile containing 500 µg per mL. Using 10 µL instead of 20 µL of each solution, proceed as directed in
Identification test
B under
Desoximetasone, beginning with “Apply separately 20 µL of each.” The specified result is observed.
pH 
791
:
between 4.0 and 8.0, in a solution prepared in the following manner. Add 15 mL of boiling water to 3.5 g of the Cream in a 50-mL centrifuge tube, cap the tube, shake vigorously until the cream is uniformly dispersed, then place the tube in a steam bath until the water and oil layers separate completely. Cool, separate the layers, and determine the pH of the aqueous phase.
Assay—
Mobile phase—
Prepare a filtered and degassed mixture of methanol, water, and glacial acetic acid (65:35:1). Make adjustments if necessary (see
System Suitability under
Chromatography 621).
Internal standard solution—
Prepare a solution of ethylparaben in methanol having a concentration of about 0.04 mg per mL.
Standard preparation—
Dissolve an accurately weighed quantity of
USP Desoximetasone RS in methanol to obtain a solution having a known concentration of about 0.4 mg per mL. Pipet 5 mL of this solution into a 50-mL centrifuge tube. Add 10.0 mL of
Internal standard solution, dilute with methanol quantitatively to 40.0 mL, and mix to obtain the
Standard preparation having a known concentration of about 0.05 mg per mL.
Assay preparation—
Transfer an accurately weighed amount of Cream, equivalent to about 2 mg of desoximetasone, to a 50-mL centrifuge tube, and add a few 3-mm glass beads. Add 10.0 mL of
Internal standard solution and about 30 mL of methanol, and mix. Tightly cap the centrifuge tube, and immerse it for 10 minutes in a bath maintained at a temperature of 65
. Remove the tube from the bath, and immediately vortex at high speed for 30 seconds. Return the tube to the hot water bath for 5 minutes, remove it from the bath, and immediately vortex for 30 seconds. Repeat the procedure one more time, then cool the tube in an ice-bath held at 10
until no further flocculent precipitation occurs. Centrifuge, and use the supernatant.
Chromatographic system
(see
Chromatography 621)—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 1 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the relative retention times are about 1 and 2 for 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 10 µL) of the
Standard preparation and the
Assay preparation into the chromatograph, 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 Cream taken by the formula:
40C(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 response ratios obtained from the
Assay preparation and the
Standard preparation, respectively.
