A: Transfer the contents of Capsules, equivalent to about 50 mg of nortriptyline hydrochloride, to a suitable flask. Add 15 mL of chloroform, insert the stopper in the flask, and shake for 15 minutes. Transfer the mixture to a suitable centrifuge tube, and centrifuge at about 2900 rpm for about 5 minutes. Pass through a suitable filter paper containing a small amount of anhydrous sodium sulfate. Evaporate the filtrate to dryness, and dissolve the residue in 0.5 mL of chloroform: the IR absorption spectrum of this solution exhibits maxima only at the same wavelengths as that of a Standard solution prepared by dissolving 50 mg of USP Nortriptyline Hydrochloride RS in 0.5 mL of chloroform.

B: A filtered solution in water of the contents of Capsules, equivalent to nortriptyline hydrochloride solution (1 in 20), responds to the tests for Chloride 191, when tested as specified for alkaloidal hydrochlorides.

Medium: water; 500 mL.

Apparatus 1: 100 rpm.

Time: 30 minutes.

Procedure— Determine the amount of C19H21N dissolved, employing the procedure set forth in the Assay, making any necessary modifications.

Tolerances— Not less than 80% (Q) of the labeled amount of C19H21N is dissolved in 30 minutes.

(Official January 1, 2007)

Phosphate buffer— Dissolve 1.63 g of monobasic potassium phosphate in 1 L of water, and adjust with 1 N potassium hydroxide to a pH of 6.7.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile, methanol, and Phosphate buffer (40:43:17). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Nortriptyline Hydrochloride RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.38 mg per mL.

Assay preparation— Weigh, empty, and combine the contents of not less than 20 Capsules. Transfer an accurately weighed portion of the powder, equivalent to about 76 mg of nortriptyline hydrochloride, to a 200-mL volumetric flask, and dissolve in about 150 mL of methanol. Shake by mechanical means for 15 minutes, dilute with methanol to volume, mix, and filter.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 239-nm detector and a 4.6-mm × 25-cm column that contains packing L10. The flow rate is about 2.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the column efficiency is not less than 500 theoretical plates, the tailing factor is not more than 3.0, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of nortriptyline (C19H21N) in the portion of Capsules taken by the formula: in which 263.38 and 299.85 are the molecular weights of nortriptyline and nortriptyline hydrochloride, respectively; C is the concentration, in mg per mL, of USP Nortriptyline Hydrochloride RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.