Identification—
Dilute the Injection with methanol, if necessary, to obtain a solution containing about 2 mg of betamethasone sodium phosphate per mL. Separately apply 10 µL of this test solution and 10 µL of a solution of
USP Betamethasone Sodium Phosphate RS in methanol containing 2 mg per mL to a thin-layer chromatographic plate (see
Chromatography 
621
) coated with chromatographic silica gel mixture. Develop the chromatogram in an equilibrated chamber containing
n-butyl alcohol previously shaken with 1 N hydrochloric acid, until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, air-dry, then spray with a mixture of sulfuric acid, methanol, and nitric acid (10:10:1). Heat the plate at 105
for 10 minutes: the
RF value of the principal spot from the test solution corresponds to that obtained from the Standard solution.
Assay—
Mobile phase—
Prepare a filtered and degassed mixture of methanol and 0.05
M monobasic potassium phosphate (1:1). Make adjustments if necessary (see
System Suitability under
Chromatography 621).
Internal standard solution—
Transfer about 100 mg of butylparaben to a 100-mL volumetric flask, add methanol to volume, and mix.
Standard preparation—
Using an accurately weighed quantity of
USP Betamethasone Sodium Phosphate RS, prepare a solution in water containing 4 mg per mL. Transfer 3.0 mL of this solution to a 25-mL volumetric flask, add 5.0 mL of
Internal standard solution, dilute with water to volume, and mix to obtain a solution having a known concentration of about 0.5 mg of
USP Betamethasone Sodium Phosphate RS per mL.
Assay preparation—
Transfer an accurately measured volume of Injection, equivalent to about 9 mg of betamethasone, to a 25-mL volumetric flask. Add 5.0 mL of the Internal standard solution, dilute with water to volume, and mix.
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 2 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the resolution,
R, between the analyte and internal standard peaks is not less than 5; and the relative standard deviation for replicate injections is not more than 2.0%.
Procedure—
Separately inject equal volumes (about 20 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 2.4 for butylparaben and 1.0 for betamethasone sodium phosphate. Calculate the quantity, in mg, of C
22H
29FO
5 in each mL of the Injection taken by the formula:
(392.47 / 516.41)(25C / V)(RU / RS),
in which 392.47 and 516.41 are the molecular weights of betamethasone and betamethasone sodium phosphate, respectively;
C is the concentration, in mg per mL, of
USP Betamethasone Sodium Phosphate RS in the
Standard preparation; V is the volume, in mL, of Injection taken; and
RU and
RS are the peak response ratios obtained from the
Assay preparation and the
Standard preparation, respectively.
meets the requirements for small-volume injections.