Medium: water; 900 mL.

Apparatus 1: 100 rpm.

Time: 45 minutes.

Procedure— Determine the amount of C18H18ClN3O dissolved from UV absorbances at 254 nm using filtered portions of the solution under test, diluted with water, if necessary, in comparison with a Standard solution having a known concentration of USP Loxapine Succinate RS in the same medium.

Tolerances— Not less than 75% (Q) of the labeled amount of C18H18ClN3O is dissolved in 45 minutes.

Mobile phase— Dissolve 4 g of tetramethylammonium chloride in 800 mL of water, add 200 mL of acetonitrile and 1 mL of phosphoric acid, mix, filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Loxapine Succinate RS in 0.01 N hydrochloric acid, and dilute quantitatively with Mobile phase to obtain a solution having a known concentration of about 0.136 mg (equivalent to 0.1 mg of loxapine base) per mL.

Assay preparation— Weigh and mix the contents of not less than 20 Capsules. Transfer an accurately weighed portion of the Capsules' contents, equivalent to 50 mg of loxapine, to a 500-mL volumetric flask, add 50 mL of 0.1 N hydrochloric acid, shake, and sonicate for 5 minutes. Dilute with Mobile phase to volume, and filter, discarding the first 8 mL of the filtrate.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 15-cm column that contains 5-µm packing L10. The flow rate is about 1.6 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the tailing factor for the analyte peak is not more than 2.0, the column efficiency determined from the analyte peak is not less than 1500 theoretical plates, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 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 C18H18ClN3O in the portion of Capsules taken by the formula: in which 327.82 and 445.91 are the molecular weights of loxapine base and loxapine succinate, respectively; C is the concentration, in mg per mL, of USP Loxapine Succinate RS in the Standard preparation, and rU and rS are the peak areas of the analyte obtained from the Assay preparation and the Standard preparation, respectively.