Buffer, Mobile phase, Diluent, and Chromatographic system— Proceed as directed in the Assay. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Use the Standard preparation, prepared as directed in the Assay.

Working standard solution— Dilute the Standard solution with Diluent, quantitatively and stepwise if necessary, to obtain a solution having a concentration of 0.625 µg per mL of citalopram hydrobromide.

System suitability solution— Dissolve an accurately weighed quantity of USP Citalopram Hydrobromide RS and USP Citalopram Related Compound D RS in Diluent, and dilute quantitatively, and stepwise if necessary, with Diluent to obtain a solution having a known concentration of about 0.001 mg per mL.

Sensitivity solution— Dilute 5.0 mL of the Working standard solution with Diluent to 50 mL to obtain a solution having 0.0625 µg of citalopram hydrobromide per mL.

Test solution— Use the Assay preparation.

Chromatographic system (see Chromatography 621)— Inject the Diluent as directed for Procedure to verify that there are no interfering peaks. Chromatograph the Sensitivity solution, and record the peak responses as directed for Procedure: the signal-to-noise ratio is at least 3. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the resolution, R, between citalopram related compound D and citalopram is not less than 1.8; the tailing factor for the citalopram hydrobromide peak is not less than 0.8 and not more than 1.5; and the relative standard deviation for replicate injections, based on the citalopram peak, is not more than 5%.

Procedure— Separately inject equal volumes (about 20 µL) of the Working standard solution and the Test solution into the chromatograph, record the chromatograms for about 40 minutes, and measure the responses for the major peaks. Calculate the percentage of related compounds in the portion of Citalopram Hydrobromide taken by the formula: in which CS and CT are the concentrations, in mg per mL, of Citalopram Hydrobromide in the Working standard solution and the Test solution, respectively; ri is the peak response for each impurity obtained from the Test solution; rS is the peak response for the citalopram peak, obtained from the Working standard solution; 324.39 and 405.30 are the molecular weights for citalopram and citalopram hydrobromide, respectively; and F is the relative response factor for each impurity relative to citalopram (free base), as presented in Table 1.

Buffer— Dissolve about 2.7 g of monobasic potassium phosphate in 1000 mL of water, add 1 mL of N,N-dimethyloctylamine, stir, and adjust with phosphoric acid to a pH of 3.0.

Diluent— Prepare a mixture of Buffer and acetonitrile (70:30).

Solution A— Prepare a mixture of Buffer, methanol, and tetrahydrofuran (70:24:6).

Solution B— Prepare a mixture of acetonitrile and Buffer (80:20).

Mobile phase— Use variable mixtures of Solution A and Solution B as directed in Table 2 for Chromatographic system. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Dissolve accurately weighed quantities of USP Citalopram Hydrobromide RS, USP Citalopram Related Compound A RS, USP Citalopram Related Compound C RS, USP Citalopram Related Compound D RS, USP Citalopram Related Compound G RS, and USP Citalopram Related Compound H RS in Diluent to obtain a final solution having a concentration of 1.5 µg per mL of each compound.

Test solution— Dissolve an accurately weighed quantity of Citalopram Hydrobromide in a suitable volume of Diluent to obtain a solution having a final concentration of 1.5 mg per mL of citalopram hydrobromide.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 224-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L1. The flow rate is about 0.8 mL per minute. The column temperature is maintained at 40. The chromatograph is programmed as shown in Table 2. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the resolution, R, between citalopram and citalopram related compound D is not less than 2.0, and that between citalopram related compound G and citalopram related compound H is not less than 4.0; and the relative standard deviation for the citalopram peak in replicate injections is not more than 2.0%.

Procedure— Inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure all the peak responses. Calculate the percentage of each citalopram related compound in the portion of Citalopram Hydrobromide taken by the formula: in which CS is the concentration, in mg per mL, of each citalopram related compound in the Standard solution; CT is the concentration of citalopram hydrobromide in the Test solution; ri is the peak area of each impurity obtained from the Test solution; rS is the peak area of each corresponding impurity obtained from the Standard solution.

100(CS / CU)(rU / rS)