A: Prepare a Standard solution of USP Tobramycin RS containing 3 mg per mL. Separately apply 6 µL of Ophthalmic Solution, 6 µL of the Standard solution, and 6 µL of a mixture consisting of equal volumes of the two solutions to a thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture. Proceed as directed for Identification test A under Tobramycin, beginning with “Place the plate in a suitable chromatographic chamber.” The specified results are obtained.

B: The retention time of the major peak for tobramycin in the chromatogram of the Derivatized assay preparation corresponds to that in the chromatogram of the Derivatized standard preparation, as obtained in the Assay.

Mobile phase , 2,4-Dinitrofluorobenzene reagent, Tris(hydroxymethyl)aminomethane reagent, and Resolution solution—Prepare as directed in the Assay under Tobramycin.

Standard preparation— Transfer about 33 mg of USP Tobramycin RS, accurately weighed, to a 50-mL volumetric flask, add 20 mL of water and 1 mL of 1 N sulfuric acid, and swirl to dissolve. Dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a second 50-mL volumetric flask, dilute with water to volume, and mix. This solution contains about 0.132 mg of USP Tobramycin RS per mL.

Assay preparation— Transfer an accurately measured volume of Ophthalmic Solution, equivalent to about 6 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 5.0 mL each of the Standard preparation and the Assay preparation, instead of 4.0 mL of each.

Chromatographic system— Proceed as directed in the Assay under Tobramycin, except to use a 4-mm × 15-cm column and to maintain the column temperature at 40.

Procedure— Proceed as directed in the Assay under Tobramycin. Calculate the quantity, in mg, of tobramycin (C18H37N5O9) in each mL of the Ophthalmic Solution taken by the formula: in which V is the volume, in mL, of Ophthalmic Solution taken to prepare the Assay preparation; and the other terms are as defined therein.