Mobile phase— Prepare a filtered and degassed mixture of water, methanol, and glacial acetic acid (89:10:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Transfer about 50 mg of USP Sulfacetamide Sodium RS, accurately weighed, to a 40-mL centrifuge tube. Add 10.0 mL of dilute methanol (1 in 5), insert the stopper, and mix, using a vortex mixer, for about 3 minutes to dissolve the Reference Standard. Add 7.5 mL of heptane, insert the stopper, and mix, using a vortex mixer, for another 3 minutes. Centrifuge to effect separation of the phases. Withdraw and discard the upper heptane layer. Transfer 3.0 mL of the bottom layer to a 500-mL volumetric flask, add dilute methanol (1 in 5) to volume, and mix.

Assay preparation— Transfer an accurately weighed quantity of Ophthalmic Ointment, equivalent to about 100 mg of sulfacetamide sodium, to a 40-mL centrifuge tube. Add 15.0 mL of heptane, insert the stopper, and mix, using a vortex mixer, for about 3 minutes to dissolve the Ophthalmic Ointment. Add 20.0 mL of dilute methanol (1 in 5), insert the stopper, and mix, using a vortex mixer, for 3 minutes. Centrifuge to effect separation of the phases. Withdraw and discard the upper heptane layer. Transfer 3.0 mL of the bottom layer to a 500-mL volumetric flask, add dilute methanol (1 in 5) to volume, and mix.

System suitability preparation— Dissolve 3 mg of sulfanilamide in 100 mL of the Standard preparation, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-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 Standard preparation and the System suitability preparation, and record the peak responses as directed for Procedure: the column efficiency determined from the analyte peak is not less than 1500 theoretical plates, the resolution, R, between the sulfacetamide and sulfanilamide peaks is not less than 3, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 90 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 C8H9N2NaO3S·H2O sulfacetamide sodium in the portion of Ophthalmic Ointment taken by the formula: in which 254.24 and 236.23 are the molecular weights of sulfacetamide sodium monohydrate and anhydrous sulfacetamide sodium, respectively, C is the concentration, in µg per mL, of sulfacetamide sodium, calculated on the anhydrous basis, in the Standard preparation, and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.