A: Infrared Absorption 197M.

B: Thin-Layer Chromatographic Identification Test 201—

C: Ignite it at 800 (see Residue on Ignition 281): the residue responds to the tests for Sodium 191 and for Phosphate 191.

Test solution: 10 mg per mL, in water.

Standard Phosphate Solution and Phosphate reagent A— Prepare as directed under Phosphate in reagents (see General Tests for Reagents under Reagents, Indicators, and Solutions).

Phosphate reagent B— Dissolve 350 mg of p-methylaminophenol sulfate in 50 mL of water, add 20 g of sodium metabisulfite, mix to dissolve, and dilute with water to 100 mL.

Procedure— Dissolve about 50 mg of Betamethasone Sodium Phosphate, accurately weighed, in a mixture of 10 mL of water and 5 mL of 2 N sulfuric acid contained in a 25-mL volumetric flask, by warming if necessary. Add 1 mL each of Phosphate reagent A and Phosphate reagent B, dilute with water to 25.0 mL, mix, and allow to stand at room temperature for 30 minutes. Similarly and concomitantly prepare a Standard solution, using 5.0 mL of Standard Phosphate Solution instead of the 50 mg of the substance under test. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 730 nm, with a suitable spectrophotometer, using water as the blank. The absorbance of the test solution is not more than that of the Standard solution. The limit is 1.0% of phosphate (PO4).

Test solution— Dissolve 25.0 mg of Betamethasone Sodium Phosphate in water to make 25.0 mL. Transfer 5.0 mL of the solution to a separator, and extract with three 25-mL portions of chloroform. Filter each extract through a chloroform-saturated cotton pledget, combining the filtrates in a conical flask. Evaporate the chloroform on a steam bath with the aid of a current of air to dryness, and dissolve the residue in methanol to make 25.0 mL.

Blank solution— Transfer 5.0 mL of water to a separator, and proceed as directed under Test solution. The methanolic solution so obtained is the Blank solution.

Procedure— Determine the absorbance (A) of the Test solution in a 1-cm cell at the wavelength of maximum absorbance at about 239 nm, with a suitable spectrophotometer, using Blank solution as the blank. Calculate the quantity, in mg, of free betamethasone in the portion of Betamethasone Sodium Phosphate taken by the formula: The limit is 250 µg (1.0%).

Mobile phase— Prepare a filtered and degassed mixture of methanol and 0.07 M anhydrous monobasic potassium phosphate (3:2). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Betamethasone Sodium Phosphate RS in a mixture of methanol and water (3:2), and dilute quantitatively, and stepwise if necessary, with the same mixture of methanol and water to obtain a solution having a known concentration of about 0.17 mg per mL.

Assay preparation— Transfer about 34 mg of Betamethasone Sodium Phosphate, accurately weighed, to a 200-mL volumetric flask, dissolve in and dilute with a mixture of methanol and water (3:2) 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 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 2, and the relative standard deviation for replicate injections is not more than 3.0%.

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. Calculate the quantity, in mg, of C22H28FNa2O8P in the portion of Betamethasone Sodium Phosphate taken by the formula: in which C is the concentration, in mg per mL, of USP Betamethasone Sodium Phosphate RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.