(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of water, methanol, and glacial acetic acid (875:125:3). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of sulfathiazole sodium in water containing about 5 mg per mL.

Standard preparation— Transfer about 25 mg of USP Sulfacetamide Sodium RS and 10 mg of p-hydroxybenzoic acid, each accurately weighed, to a 100-mL volumetric flask. Dissolve in water, add 5 mL of Internal standard solution, dilute with water to volume, and mix. Transfer 2 mL of this solution to a 25-mL volumetric flask, dilute with water to volume, and mix.

Assay preparation— Transfer about 250 mg of Topical Suspension, accurately weighed, to a 125-mL conical flask, add 5 mL of Internal standard solution, dilute with 95 mL of water, and mix. Transfer 2 mL of this solution to a 25-mL volumetric flask, dilute with water to volume, and mix. Centrifuge, and use the clear supernatant.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 2.0 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.2 for sulfanilamide, 0.5 for sulfacetamide, 1.0 for sulfathiazole, and 1.2 for p-hydroxybenzoic acid; the resolution, R, between sulfathiazole and p-hydroxybenzoic acid is not less than 2.0; the tailing factor is not more than 2; 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 major peaks. Calculate the quantity, in mg, of sulfacetamide sodium (C8H9N2NaO3S) in the portion of Topical Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Sulfacetamide Sodium RS in the Standard preparation; and RU and RS are the peak area ratios of sulfacetamide to sulfathiazole obtained from the Assay preparation and the Standard preparation, respectively.