Identification—
Evaporate 9 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 a mixture of methylene chloride and methanol (1:1). Apply separately 5 µL of this solution and 5 µL of a solution of Dexamethasone RS in the mixture of methylene chloride and methanol (1:1), containing 0.5 mg per mL, to a thin-layer chromatographic plate coated with a 0.25-mm layer of chromatographic silica gel mixture (see
Chromatography 
621
). Allow the spots to dry, and develop the chromatogram in a solvent system consisting of a mixture of chloroform, acetone, and glacial acetic acid (80:40:1) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots by viewing under short-wavelength UV light: the
RF value of the principal spot obtained from the solution under test corresponds to that obtained from the Standard solution.
Assay—
Mobile phase—
Prepare a filtered and degassed mixture of water and acetonitrile (2:1). Make adjustments if necessary (see
System Suitability under
Chromatography 621).
Standard preparation—
Dissolve an accurately weighed quantity of
USP Dexamethasone RS in dilute methanol (1 in 2), and dilute quantitatively, and stepwise if necessary, with dilute methanol (1 in 2) to obtain a solution having a known concentration of about 0.1 mg per mL.
Assay preparation—
Transfer an accurately measured volume of Elixir, freshly mixed and free from air bubbles, equivalent to about 1 mg of dexamethasone, to a 10-mL volumetric flask, dilute with water to volume, mix, and filter through a suitable membrane filter.
Chromatographic system
(see
Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-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 under
Procedure: the relative standard deviation for replicate injections is not more than 3.0%.
Procedure—
Separately inject equal volumes (between 5 µL and 25 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 each mL of the Elixir taken by the formula:
10(C / V)(rU / rS),
in which
C is the concentration, in mg per mL, of
USP Dexamethasone RS in the
Standard preparation,
V is the volume, in mL, of Elixir taken, and
rU and
rS are the peak responses obtained from the
Assay preparation and the
Standard preparation, respectively.
