A: Reduce the contents of a number of Capsules,equivalent to 20mg of trihexyphenidyl hydrochloride,to a fine powder,and triturate with 25mLof chloroform.Filter the mixture,and evaporate the filtrate,by gently heating,to about 10mL.Add the solution to 100mLof n-hexane:a white precipitate is formed.Allow the mixture to stand for 30minutes,and collect the precipitate on a solvent-resistant membrane filter to 1-µm pore size.Wash the crystals with a small portion of n-hexane,and air-dry:the IRabsorption spectrum of a potassium bromide dispersion of the crystals so obtained exhibits maxima only at the same wavelengths as that of a similar preparation of USP Trihexyphenidyl Hydrochloride RS.

B: The precipitate obtained in Identificationtest Aresponds to the tests for Chloride á191ñ.

Medium: water;500mL.

Apparatus 1: 100rpm.

Time: 3hours;6hours;12hours.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile,water,and triethylamine (920:80:0.2),and adjust with phosphoric acid to a pHof 4.0.

Standard preparation— Dissolve an accurately weighed quantity of USP Trihexyphenidyl Hydrochloride RSin water,and dilute quantitatively and stepwise with water to obtain a solution having a known concentration of about 5µg per mL.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm ×8.3-cm column that contains packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the tailing factor for the trihexyphenidyl peak is not more than 2.8,and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Dilute the Standard preparationand the solution under test with acetonitrile (1:1).Separately inject equal volumes (about 20µL)of these solutions into the chromatograph,record the chromatograms,and measure the responses for trihexyphenidyl.Calculate the percentage of the labeled amount of C20H31NO·HCl dissolved.

Tolerances— The percentages of the labeled amount of C20H31NO·HCl dissolved at the times specified conform to Acceptance Table 1.

Mobile phase and Chromatographic system—Prepare as directed in the Assayunder Trihexyphenidyl Hydrochloride.

Standard preparation— Dissolve an accurately weighed quantity of USP Trihexyphenidyl Hydrochloride RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 0.2mg per mL.

Assay preparation— Fill a 500-mLvolumetric flask with Mobile phaseto volume.Transfer to this flask the accurately weighed contents of a counted number of Capsules,equivalent to about 100mg of trihexyphenidyl hydrochloride.Mix,sonicate,with occasional shaking for 45minutes,allow to stand for 15minutes,and filter,discarding the first 5mLof the filtrate.

Procedure— Proceed as directed for Procedurein the Assayunder Trihexyphenidyl Hydrochloride.Calculate the quantity,in mg,of C20H31NO·HCl in each of the Capsules taken by the formula: in which Nis the number of Capsules taken,and the other terms are as defined therein.