A: To 1 mL of Ophthalmic Suspension in a test tube, add 100 mg of sodium sulfate, disperse by shaking, and centrifuge: the clear supernatant meets the requirements for Identification test A under Tobramycin.

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.

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 Suspension, equivalent to about 4.5 mg of tobramycin, to a 50-mL volumetric flask, dilute with water to volume, and mix.

Derivatization procedure— Proceed as directed in the Assay under Tobramycin, except to use 10.0 mL of the Assay preparation instead of 4.0 mL.

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

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

Standard preparation— Transfer about 25 mg of USP Dexamethasone RS, accurately weighed, to a 25-mL volumetric flask, dissolve in methanol, dilute with methanol to volume, and mix. Transfer 4.0 mL of this solution to a 100-mL volumetric flask, dilute with methanol to volume, and mix. This solution contains about 0.04 mg of USP Dexamethasone RS per mL.

Assay preparation— Transfer an accurately measured volume of Ophthalmic Suspension, freshly mixed and free from air bubbles, equivalent to about 4 mg of dexamethasone, to a 100-mL volumetric flask, dilute with methanol to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 25-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and measure the peak responses as directed for Procedure: the tailing factor for the analyte peak is not more than 1.5, the column efficiency is not less than 1400 theoretical plates, 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. Calculate the quantity, in mg, of dexamethasone (C22H29FO5), in each mL of the Ophthalmic Suspension taken by the formula: 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 Ophthalmic Suspension taken, and rU and rS are the dexamethasone peak responses obtained from the Assay preparation and the Standard preparation, respectively.