A: To 1 g of Ophthalmic Ointment in a test tube add 2 mL of chloroform, and shake to dissolve. Add 0.5 mL of sodium sulfate solution (1 in 10), shake vigorously, and centrifuge: the clear supernatant aqueous liquid meets the requirements for Identification test A under Tobramycin. [NOTE—If, after centrifuging, an oily film remains on top of the supernatant aqueous liquid, transfer the supernatant aqueous liquid to a second test tube, and wash it with 2 mL of chloroform.]

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 , 2,4-Dinitrofluorobenzene reagent, Tris(hydroxymethyl)aminomethane reagent, Standard preparation, Resolution solution, and Chromatographic system—Proceed as directed in the Assay under Tobramycin.

Assay preparation— Transfer an accurately weighed portion of Ophthalmic Ointment, equivalent to about 4.5 mg of tobramycin, to a separator, add 50 mL of ether, and extract with four 20- to 25-mL portions of water. Combine the water extracts in a 100-mL volumetric flask, dilute with water to volume, and mix.

Derivatization procedure— Proceed as directed in the Assay under Tobramycin, except to use 15.0 mL of Assay preparation instead of 4.0 mL.

Procedure— Proceed as directed in the Assay under Tobramycin. Calculate the quantity of tobramycin (C18H37N5O9), in mg, in the portion of Ophthalmic Ointment taken by the formula: in which the terms are as defined therein.

Diluent— Prepare a mixture of methanol and water (750:250).

Mobile phase— Prepare a suitable mixture of methanol and water (55:45), pass through a suitable filter having a 1-µm or finer porosity, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Dexamethasone RS in Diluent to obtain a stock solution having a known concentration of about 0.2 mg per mL. Transfer 15.0 mL of this stock solution to a separator containing about 50 mL of n-hexane, and shake. Allow the layers to separate, and drain the lower phase into a 50-mL volumetric flask. Repeat the extraction with two 15-mL portions of Diluent, combining the lower phase from each extraction in the same 50-mL volumetric flask. Dilute with Diluent to volume, and mix. This solution contains about 0.06 mg of USP Dexamethasone RS per mL.

Resolution solution— Prepare a stock solution of chlorobutanol and USP Dexamethasone RS in Diluent containing about 1 mg of anhydrous chlorobutanol and 0.2 mg of USP Dexamethasone RS per mL. Proceed as directed for Standard preparation beginning with “Transfer 15.0 mL of this stock solution to a separator.” The solution so obtained contains about 0.3 mg of anhydrous chlorobutanol and 0.06 mg of USP Dexamethasone RS per mL.

Assay preparation— Transfer an accurately weighed quantity of Ophthalmic Ointment, equivalent to about 3 mg of dexamethasone, to a separator containing about 50 mL of n-hexane, and shake. Add 15 mL of Diluent, and shake. Allow the layers to separate, and drain the lower phase into a 50-mL volumetric flask. Repeat the extraction with two 15-mL portions of Diluent, combining the lower phase from each extraction in the same 50-mL volumetric flask. Dilute with Diluent to volume, mix, and centrifuge. Use the clear solution.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 206-nm detector and an 8.0-mm × 10-cm column that contains packing L1. The flow rate is about 3 mL per minute. Chromatograph the Resolution solution, and measure the peak responses as directed for Procedure: the relative retention times are about 0.7 for chlorobutanol and 1.0 for dexamethasone; and the resolution, R, between chlorobutanol and dexamethasone is not less than 1.8. Chromatograph the Standard preparation, and measure the peak responses as directed for Procedure: the tailing factor is not more than 2; the column efficiency is not less than 350 theoretical plates; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 100 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 dexamethasone (C22H29FO5) in the portion of Ophthalmic Ointment taken by the formula: in which C is the concentration, in mg per mL, of USP Dexamethasone RS in the Standard preparation; and rU and rS are the dexamethasone peak responses obtained from the Assay preparation and the Standard preparation, respectively.