(Official January 1, 2007)

Mobile phase— Prepare a 0.005 M tetrabutylammonium phosphate solution, and adjust with phosphoric acid to a pH of 2.0. Prepare a filtered and degassed mixture of this solution and methanol (750:250). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Ciprofloxacin Hydrochloride RS in 0.1 N hydrochloric acid to obtain a solution having a known concentration of about 0.033 mg per mL.

Resolution solution— Dissolve an accurately weighed quantity of USP Ciprofloxacin Ethylenediamine Analog RS in a portion of the Standard preparation to obtain a solution containing about 0.005 mg per mL.

Assay preparation— Transfer an accurately weighed quantity of Ophthalmic Ointment, equivalent to about 750 µg of ciprofloxacin, to a screw-capped tube. Add 15 mL of solvent hexane, and shake vigorously until the Ophthalmic Ointment is dispersed. Loosen the cap, and heat in a water bath at 60 for 30 minutes, with occasional swirling. Remove from the bath, tighten the cap, and shake for 1.5 minutes while still hot. Add 25.0 mL of 0.1 N hydrochloric acid, and shake vigorously for 1.5 minutes. Allow the layers to separate, and use the lower, aqueous layer.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Resolution solution, and record the responses as directed for Procedure: the relative retention times are about 0.8 for the ciprofloxacin ethylenediamine analog and 1.0 for ciprofloxacin; and the resolution, R, between ciprofloxacin ethylenediamine analog and ciprofloxacin is not less than 2.0. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the column efficiency is not less than 500 theoretical plates; the tailing factor is not less than 0.9 and not more than 2.0; 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 areas for the ciprofloxacin peaks. Calculate the quantity, in mg, of ciprofloxacin (C17H18FN3O3) in each g of the Ophthalmic Ointment taken by the formula: in which 331.34 and 385.82 are the molecular weights of ciprofloxacin and ciprofloxacin hydrochloride monohydrate, respectively; C is the concentration, in mg per mL, of USP Ciprofloxacin Hydrochloride RS in the Standard preparation; W is the weight, in g, of Ophthalmic Ointment taken; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.