Specific rotation 
781S
:
between +118
and +126
, calculated on the dried basis.
Test solution:
5 mg per mL, in methanol.
Ordinary impurities 466—
Test solution:
methanol.
Standard solution:
methanol.
Application volume:
10 µL.
Eluant:
a mixture of toluene, acetone, methyl ethyl ketone, and formic acid (55:20:20:5), in a nonequilibrated chamber.
Visualization:
5.
Organic volatile impurities, Method V 467:
meets the requirements, except that the limit for methylene chloride is 0.1%.
Solvent—
Use dimethyl sulfoxide.
Assay—
Mobile phase—
Prepare a filtered and degassed mixture of water and acetonitrile (63:37). Make adjustments if necessary (see
System Suitability under
Chromatography 621).
Internal standard solution—
Prepare a solution of propylparaben in alcohol having a known concentration of about 0.25 mg per mL.
Standard preparation—
Dissolve an accurately weighed quantity of
USP Betamethasone RS in alcohol to obtain a solution having a known concentration of about 0.2 mg per mL. Transfer 10.0 mL of this solution to a suitable vial, and add 10.0 mL of
Internal standard solution, to obtain a
Standard preparation having known concentrations of about 0.1 mg of betamethasone and about 0.125 mg of propylparaben per mL.
Assay preparation—
Using about 80 mg of Betamethasone, accurately weighed, prepare as directed for Standard preparation.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a 240-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1.0 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the relative retention times are about 1.0 for betamethasone and 1.4 for propylparaben; the resolution,
R, between betamethasone and propylparaben is not less than 3.0; 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
5 in the portion of Betamethasone taken by the formula:
800C(RU / RS),
in which
C is the concentration, in mg per mL, of
USP Betamethasone RS in the
Standard preparation; and
R U and
R S are the peak height ratios of the betamethasone peak and the internal standard peak obtained from the
Assay preparation and the
Standard preparation, respectively.
;)
,16