A: Reduce the contents of a number of Capsules, equivalent to 20 mg of trihexyphenidyl hydrochloride, to a fine powder, and triturate with 25 mL of chloroform. Filter the mixture, and evaporate the filtrate, by gently heating, to about 10 mL. Add the solution to 100 mL of n-hexane: a white precipitate is formed. Allow the mixture to stand for 30 minutes, 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 IR absorption 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 Identification test A responds to the tests for Chloride 191.

Medium: water; 500 mL.

Apparatus 1: 100 rpm.

Time: 3, 6, and 12 hours.

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

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

Chromatographic system (see Chromatography 621)— 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 2 mL per 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 preparation and the solution under test with acetonitrile (1:1). Separately inject equal volumes (about 20 µL) 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.

Medium: water; 500 mL.

Apparatus 1: 100 rpm.

Times: 3, 6, and 12 hours.

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

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

Chromatographic system (see Chromatography 621)—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 2 mL per minute. Chromatograph the Standard solution, 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 solution and the solution under test with acetonitrile (1:1). Separately inject equal volumes (about 20 µL) of these solutions into the chromatograph, record the chromatograms, and measure the peak 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 2.

(Official April 1, 2006)

(Official January 1, 2007)

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

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

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

Procedure— Proceed as directed for Procedure in the Assay under Trihexyphenidyl Hydrochloride. Calculate the quantity, in mg, of trihexyphenidyl hydrochloride (C20H31NO·HCl) in each of the Capsules taken by the formula: in which N is the number of Capsules taken, and the other terms are as defined therein.