Mobile phase— Prepare a filtered and degassed mixture of water,methanol,and glacial acetic acid (875:125:3).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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

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

Chromatographic system (see Chromatography á621ñ)—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 2.0mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.2for sulfanilamide,0.5for sulfacetamide,1.0for sulfathiazole,and 1.2for 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 20µL)of the Standard preparationand the Assay preparationinto 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 Cis the concentration,in mg per mL,of USP Sulfacetamide Sodium RSin the Standard preparation;and RUand RSare the peak area ratios of sulfacetamide to sulfathiazole obtained from the Assay preparationand the Standard preparation,respectively.