Mobile phase— Adjust about 1000 mL of Buffer No. 1 (see Phosphate Buffers and Other Solutions in the section Media and Diluents, under Antibiotics—Microbial Assays 81) with 10 N potassium hydroxide to a pH of 6.6. To about 35 volumes of this solution add about 65 volumes of methanol, mix, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve accurately weighed quantities of USP Hydroxyzine Hydrochloride RS and 4-chlorobenzophenone in Mobile phase, and dilute quantitatively with Mobile phase to obtain a solution having known concentrations of about 250 µg of USP Hydroxyzine Hydrochloride RS and 0.5 µg of 4-chlorobenzophenone per mL. Protect this solution from light.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 125 mg of hydroxyzine hydrochloride, to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix. Pipet 10 mL of this solution into a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix. Protect this solution from light.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4-mm × 30-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 under Procedure: the tailing factors for the 4-chlorobenzophenone and hydroxyzine peaks are not more than 2.5, the resolution, R, between the 4-chlorobenzophenone and hydroxyzine peaks is not less than 2.0, and the relative standard deviation for replicate injections of the Standard preparation is not more than 2.0%. The relative retention times are about 0.75 for 4-chlorobenzophenone and 1.0 for hydroxyzine.

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 hydroxyzine hydrochloride (C21H27ClN2O2·HCl) in each mL of the Injection taken by the formula: in which C is the concentration, in µg per mL, of USP Hydroxyzine Hydrochloride RS in the Standard preparation; V is the volume, in mL, of Injection taken; and rU and rS are the hydroxyzine peak responses obtained from the Assay preparation and the Standard preparation, respectively. The ratio of the response of the 4-chlorobenzophenone peak to that of the hydroxyzine peak obtained from the Assay preparation does not exceed the corresponding ratio of peak responses obtained from the Standard preparation (0.2%).