A: It meets the requirements under Thin-Layer Chromatographic Identification Test 201BNP.

B: The retention time of the major peak for dexamethasone in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay for dexamethasone.

(Official January 1, 2007)

Mobile phase— Prepare a suitable aqueous solution of acetonitrile, approximately 1 in 3, such that the retention time of dexamethasone is about 5 minutes.

Standard preparation— Dissolve an accurately weighed quantity of USP Dexamethasone RS in a mixture of acetonitrile and methanol (1:1) to obtain a solution having a known concentration of about 60 µg per mL.

Assay preparation— Transfer an accurately weighed portion of Ophthalmic Ointment, equivalent to about 3 mg of dexamethasone, to a suitable test tube, add about 15 mL of cyclohexane, and heat in a water bath at 75 ± 5 for 10 minutes. [NOTE—If the ointment is not fully dissolved, heat on a steam bath for about 30 seconds, place a cap on the test tube, and place on a vortex mixer until all solid material is dissolved.] Filter with suction through a medium-porosity sintered-glass filter. Rinse the test tube twice with 10-mL portions of cyclohexane, filtering the rinsings through the filter, and discard the filtrates. Wash the filter with about 10 mL of a mixture of acetonitrile and methanol (1:1), and collect the filtrate in a 50-mL beaker. Wash the test tube and the filter with several 10-mL portions of the same solvent, and combine the washings in the 50-mL beaker. Transfer the contents of the beaker to a 50-mL volumetric flask, with the aid of a mixture of acetonitrile and methanol (1:1), dilute with the same solvent to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains 5- to 10-µm packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak response as directed under Procedure: the column efficiency is not less than 4000 theoretical plates, and the relative standard deviation for replicate injections is not more than 1.5%.

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 C22H29FO5 in the portion of Ophthalmic Ointment taken by the formula: in which C is the concentration, in µg per mL, of USP Dexamethasone RS in the Standard preparation; and rU and rS are the peak responses of the Assay preparation and the Standard preparation, respectively.