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

B: The retention time of the major peak for dexamethasone in the chromatogram of the Assay preparationcorresponds 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 Assayunder Tobramycin.

Assay preparation— Transfer an accurately weighed portion of Ophthalmic Suspension,equivalent to about 4.5mg of tobramycin,to a 50-mLvolumetric flask,dilute with water to volume,and mix.

Derivatization procedure— Proceed as directed in the Assayunder Tobramycin,except to use 10.0mLof the Assay preparationinstead of 4.0mL.

Procedure— Proceed as directed in the Assayunder 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 1µm or less,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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

Chromatographic system (see Chromatography á621ñ)—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 1.5mLper 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 1400theoretical plates,and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of dexamethasone (C22H29FO5),in each mLof the Ophthalmic Suspension taken by the formula: in which Cis the concentration,in mg per mL,of USP Dexamethasone RSin the Standard preparation,Vis the volume,in mL,of Ophthalmic Suspension taken,and rUand rSare the dexamethasone peak responses obtained from the Assay preparationand the Standard preparation,respectively.