Identification—
Transfer a quantity of Ointment, equivalent to about 1.0 mg of clobetasol propionate, to a 25-mL plastic-stoppered centrifuge tube. Add 10 mL of methanol, and cap. Heat in a 70
water bath for about 4 minutes, remove the tube from the bath, and shake vigorously. Repeat the heating and shaking. Freeze the mixture in an ice bath for about 5 minutes, and centrifuge at about 3500 rpm for about 10 minutes. Transfer about 5 mL of the supernatant to a suitable vial. Evaporate with the aid of a stream of nitrogen to dryness. Dissolve the residue in about 1.0 mL of chloroform to obtain the test solution. Prepare a Standard solution of
USP Clobetasol Propionate RS having the same concentration as the test solution. The test solution so obtained responds to the
Thin-Layer Chromatographic Identification Test 
201
, a mixture of chloroform, acetone, and ethanol (100:10:5) being used as the developing solvent.
Microbial limits 61—
It meets the requirements of the tests for absence of
Staphylococcus aureus,
Pseudomonas aeruginosa,
Escherichia coli, and
Salmonella species, and the total aerobic microbial count does not exceed 100 cfu per g.
Assay—
[NOTE—Where peak responses are indicated, use peak areas.
]
Mobile phase, Internal standard solution, Standard preparation, System suitability solution, and Chromatographic system—
Proceed as directed in the
Assay under
Clobetasol Propionate.
Assay preparation—
Transfer an accurately weighed portion of Ointment, equivalent to about 1.0 mg of clobetasol propionate, to a 125-mL separator. Add 30 mL of hexane, 10.0 mL of Internal standard solution, and shake. Collect the lower layer in a 25-mL volumetric flask. Extract the hexane remaining in the separator with two 5-mL portions of Mobile phase, and combine all of the extracts in the 25-mL volumetric flask. Dilute with Mobile phase to volume, and mix. Filter a portion through a 0.45-µm filter.
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. The relative retention times are about 1.0 for clobetasol propionate and 1.6 for beclomethasone dipropionate. Calculate the quantity, in mg, of C
25H
32ClFO
5 in the portion of Ointment taken by the formula:
25C(RU / RS),
in which
C is the concentration, in mg per mL, of
USP Clobetasol Propionate RS in the
Standard preparation; and
RU and
RS are the ratios of the clobetasol propionate peak to the internal standard peak obtained from the
Assay preparation and the
Standard preparation, respectively.
