Mobile phase— Prepare a filtered and degassed mixture of methanol and 0.01Mmonobasic potassium phosphate (60:40).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Internal standard solution— Dissolve a suitable quantity of ethylparaben in methanol,and dilute with an equal volume of water to obtain a solution containing about 5.0mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Bisacodyl RSin methanol,add an accurately measured volume of Internal standard solution,and dilute quantitatively,and stepwise if necessary,with methanol to obtain a solution having known concentrations of about 67µg per mLand 250µg per mLfor bisacodyl and ethylparaben,respectively.

Assay preparation— Transfer an accurately measured volume of Rectal Suspension,equivalent to 6.7mg of bisacodyl,to a 100-mLvolumetric flask.Add 5.0mLof Internal standard solution,dilute with methanol to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm ×30-cm column containing packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 2.0for bisacodyl and 1.0for ethylparaben;the resolution,R,between bisacodyl and the internal standard is not less than 7.0;the column efficiency,determined for the analyte peak,is not less than 2000theoretical plates;the tailing factor is not more than 1.2;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C22H19NO4in the portion of Rectal Suspension taken by the formula: in which Cis the concentration,in mg per mL,of USP Bisacodyl RSin the Standard preparation;and RUand RSare the peak response ratios of the bisacodyl peak to the internal standard peak obtained from the Assay preparationand the Standard preparation,respectively.