Triethanolamine phosphate solution— Dissolve 2mLof triethanolamine in about 900mLof water in a 1000-mLvolumetric flask,adjust with phosphoric acid to a pHof 3.5,dilute with water to volume,and mix.

Mobile phase— Prepare a degassed and filtered mixture of acetonitrile and Triethanolamine phosphate solution(65:35).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Prepare a solution containing 2.0mg of USP Oxybutynin Chloride RSper mLand 10µg of USP Phenylcyclohexylglycolic Acid RSper mLin acetonitrile.Filter through a 0.45-µm filter.

Test preparation— Transfer 200mg of Oxybutynin Chloride,accurately weighed,to a 100-mLvolumetric flask,dissolve in acetonitrile,dilute with acetonitrile to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 200-nm detector and a 4.6-mm ×25-cm column that contains packing L7,and a short pre-column that contains packing L7is used if necessary.The column is maintained at a temperature of 45.The flow rate is about 3mLper minute.Chromatograph a solution in acetonitrile containing 40µg per mLeach of USP Oxybutynin Chloride RSand USP Phenylcyclohexylglycolic Acid RS,and record the peak responses as directed for Procedure:the relative standard deviation of the responses for replicate injections is not more than 3.0%,and the resolution,R,of the main peaks is not less than 5.0.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Test preparationinto the chromatograph,and record the chromatograms.The relative retention times are about 0.2for phenylcyclohexylglycolic acid and 1.0for oxybutynin.In the chromatogram of the Test preparation,any peak corresponding in retention time to phenylcyclohexylglycolic acid has a response not greater than that in the chromatogram of the Standard preparation,and the sum of the responses of all peaks,excluding the solvent front and oxybutynin,is not greater than twice that value.