Internal standard solution— Dissolve butyl benzoate in methanol to obtain a solution containing about 17.5mg per mL.

Mobile phase— Prepare a suitable degassed and filtered mixture of acetonitrile and water (3:2).

Standard solution— Dissolve a suitable quantity of USP Crotamiton RS,accurately weighed,in methanol to obtain a solution having a known concentration of about 1mg per mL.

Standard preparation— Pipet 10mLof Standard solutionand 5mLof Internal standard solutioninto a 50-mLvolumetric flask,dilute with methanol to volume,and mix.

Assay preparation— Transfer an accurately weighed portion of Crotamiton Cream,equivalent to about 50mg of crotamiton,to a tared 50-mLvolumetric flask.Add about 25mLof methanol,and shake and sonicate to disperse the cream.Dilute with methanol to volume,and mix.Filter about 20mLthrough moderately retentive filter paper.Pipet 10mLof the clear filtrate and 5mLof Internal standard solutioninto a 50-mLvolumetric flask,dilute with methanol to volume,and mix.

Procedure— Inject equal volumes of the Standard preparationand the Assay preparationinto a liquid chromatograph (see Chromatography á621ñ)equipped with a 254-nm detector and a 4.6-mm ×25-cm stainless steel column that contains packing L1.In a suitable chromatogram,the resolution,R,between peaks for crotamiton and butyl benzoate is not less than 3.0;and the lowest and highest peak response ratios (RS)of three replicate injections of the Standard preparationagree within 2.0%.Calculate the quantity,in mg,of C13H17NOin the portion of Cream taken by the formula: in which Cis the concentration,in mg per mL,of USP Crotamiton RSin the Standard preparation;and RUand RSare the peak response ratios of the crotamiton peak and the butyl benzoate peak obtained from the Assay preparationand the Standard preparation,respectively.