Mobile phase— Prepare a solution of sodium acetate in water containing 2.5mg per mL,and adjust with glacial acetic acid to a pHof 4.75±0.1.Prepare a mixture of this solution and acetonitrile (800:225).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

System suitability solution— Prepare a solution in Mobile phasecontaining in each mLabout 1.2µg of methylparaben,12µg of sulfabenzamide,and 72µg of USP Oxfendazole RS.

Internal standard solution— Prepare a solution of sulfabenzamide in Mobile phasecontaining about 0.3mg per mL.

Standard preparation— Prepare a solution of USP Oxfendazole RSin methanol having a known concentration of about 900µg per mL.Transfer 20.0mLof this solution to a 100-mLvolumetric flask,add 4.0mLof Internal standard solution,dilute with Mobile phaseto volume,and mix.This solution contains about 180µg of USP Oxfendazole RSper mL.

Assay preparation— Transfer an accurately measured volume of the Suspension,previously well-mixed and free from air bubbles,equivalent to about 450mg of oxfendazole,to a 500-mLvolumetric flask.Add about 30mLof water,and swirl to disperse.Add about 300mLof methanol,and dissolve with the aid of sonication.Transfer 20.0mLof this solution to a 100-mLvolumetric flask,add 4.0mLof Internal standard solution,dilute with Mobile phaseto volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 254-nm detector,a guard column containing packing L2,and a 4.6-mm ×25-cm analytical column that contains packing L1.The flow rate is about 1mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.7for sulfabenzamide,0.8for methylparaben,and 1.0for oxfendazole;the resolution,R,between the methylparaben peak and the oxfendazole peak is not less than 2.0;the column efficiency determined from the oxfendazole peak is not less than 2000theoretical plates;and the relative standard deviation of replicate injections is not more than 1.5%.[NOTE—The detector sensitivity may be changed between the peaks to keep the responses on scale.]

Procedure— Separately inject equal volumes (about 50µL)of the Standard preparationand the Assay preparationinto the chromatograph,and measure the responses for the major peaks.Calculate the quantity,in mg,of oxfendazole (C15H13N3O3S)in each mLof the Suspension taken by the formula: in which Cis the concentration,in µg per mL,of USP Oxfendazole RSin the Standard preparation;Vis the volume,in mL,of Suspension taken to prepare the Assay preparation;and RUand RSare the ratios of the oxfendazole peak response to the sulfabenzamide peak response obtained from the Assay preparationand the Standard preparation,respectively.