Medium: 0.1 N hydrochloric acid; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes for hydrochlorothiazide, 30 minutes for lisinopril.

Buffer solution and Mobile phase— Proceed as directed in the Assay.

Lisinopril standard solution— Dissolve an accurately weighed quantity of USP Lisinopril RS in Buffer solution to obtain a solution having a known concentration of about 0.5 mg per mL.

Hydrochlorothiazide standard solution— Dissolve an accurately weighed quantity of USP Hydrochlorothiazide RS in methanol to obtain a solution having a known concentration of about 3.12 mg per mL. Transfer 7 mL of this solution to a 50-mL volumetric flask, and dilute with methanol to volume.

Working standard solution— Transfer an appropriate aliquot of Lisinopril standard solution and Hydrochlorothiazide standard solution into a suitable volumetric flask, and dilute with Medium to volume to obtain a solution containing known concentrations of lisinopril and hydrochlorothiazide as mentioned in Table 1.

Test solution— Pass a portion of the solution under test through a suitable 0.45-µm filter, discarding the first few mL of the filtrate.

Chromatographic system (see Chromatography 621)— Proceed as directed in the Assay. Chromatograph the Working standard solution, and record the peak responses as directed for Procedure: the tailing factor for both lisinopril and hydrochlorothiazide peaks is less than 2; the theoretical plate count for the hydrochlorothiazide peak is greater than 6000; the resolution, R, between the lisinopril and hydrochlorothiazide peaks is greater than 4.0; and the relative standard deviation for replicate injections is not more than 2.0% for both lisinopril and hydrochlorothiazide peaks.

Procedure— Separately inject equal volumes (about 20 µL) of the Working standard solution and the Test solution into the chromatograph, record the chromatograms, measure the response for the major peaks, and determine the amount, in percentage, of C21H31N3O5 or C7H8ClN3O4 S2 dissolved by the formula: in which rU and rS are the peak responses obtained from the Test solution and the Working standard solution, respectively; CS is the concentration, in mg per mL, of lisinopril and hydrochlorothiazide as mentioned in Table 1; and L is the Tablet label claim of lisinopril or hydrochlorothiazide, in mg.

Tolerances— Not less than 80% (Q) of the labeled amount of C21H31N3O5 is dissolved in 30 minutes. Not less than 80% (Q) of C7H8ClN3O4 S2 is dissolved in 45 minutes.

Buffer solution, Mobile phase, Benzothiadiazine related compound A standard preparation, and Chromatographic system— Proceed as directed in the Assay.

Standard solution 1— Use the Combined standard preparation, prepared as directed in the Assay.

Standard solution 2— Transfer 2 mL of Benzothiadiazine related compound A standard preparation into a 25-mL volumetric flask, and dilute with Buffer solution to volume.

Test solution— Use the Assay preparation.

Procedure— Proceed as directed in the Assay. Separately inject a volume (about 10 µL) of Standard solution 1 and Standard solution 2 into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the percentage of lisinopril related compound A and benzothiadiazine related compound A in the portion of Tablets taken by the formula: in which CS is the concentration, in mg per mL, of USP Lisinopril Related Compound A RS in Standard solution 1 or USP Benzothiadiazine Related Compound A RS in Standard solution 2; CU is the concentration, in mg per mL, of lisinopril or hydrochlorothiazide in the Test solution; rU and rS are the peak responses of the lisinopril related compound A or benzothiadiazine related compound A obtained from the Test solution and Standard solution 1 or Standard solution 2, respectively; and MW1 is the molecular weight of lisinopril (405.5) [use for calculating the percentage of lisinopril related compound A] or hydrochlorothiazide (297.74) [use for calculating the percentage of benzothiadiazine related compound A]; MW2 is the molecular weight of the lisinopril related compound A (387.5) or benzothiadiazine related compound A (285.74). Not more than 2% of lisinopril related compound A is found and not more than 1% of benzothiadiazine related compound A is found.

Buffer solution— Dissolve 4.08 g of monobasic potassium phosphate in 800 mL of water. Adjust with phosphoric acid to a pH of 2.5, dilute with water to 1 L, and mix well.

Mobile phase— Prepare a filtered mixture of water, acetonitrile, phosphoric acid, and triethylamine (1480: 280:15:3). Make adjustments if necessary (see System Suitability under Chromatography 621).

Lisinopril standard preparation— Dissolve an accurately weighed quantity of USP Lisinopril RS in Buffer solution to obtain a solution having a known concentration of about 0.5 mg per mL.

Hydrochlorothiazide standard preparation— Dissolve an accurately weighed quantity of USP Hydrochlorothiazide RS in methanol to obtain a solution having a known concentration of about 3.12 mg per mL.

Lisinopril related compound A standard preparation— Dissolve an accurately weighed quantity of USP Lisinopril Related Compound A RS in methanol to obtain a solution having a known concentration of about 0.05 mg per mL.

Benzothiadiazine related compound A standard preparation— Dissolve an accurately weighed quantity of USP Benzothiadiazine Related Compound A RS in methanol to obtain a solution having a known concentration of about 0.016 mg per mL.

Combined standard preparation— Dilute appropriate amounts of each of the above mentioned standard preparations quantitatively, and stepwise if necessary, with Buffer solution to obtain a Combined standard solution having a known concentration of about 0.1 mg per mL of USP Lisinopril RS; 0.125 mg per mL of USP Hydrochlorothiazide RS; 0.002 mg per mL of USP Lisinopril Related Compound A RS; and 0.0013 mg per mL of USP Benzothiadiazine Related Compound A RS. [Note—In the case of Tablet strengths 20/12.5 of lisinopril and hydrochlorothiazide, the concentration of lisinopril and lisinopril related compound A in the Combined standard solution is 0.2 and 0.004 mg per mL, respectively.]

Assay stock preparation— Transfer 10 Tablets to a suitable volumetric flask, add Buffer solution (0.25 mL per mg of total lisinopril), sonicate for 5 minutes, and then add methanol (0.5 mL per mg of total lisinopril). Sonicate for an additional 10 minutes. Add more Buffer solution (0.75 mL per mg of total lisinopril), and mix using a mechanical shaker for 20 minutes. Dilute with water to volume to obtain a solution having a known concentration of 0.4 mg per mL of lisinopril and 0.5 mg per mL of hydrochlorothiazide for Tablet strengths of 10/12.5 and 20/25 of lisinopril and hydrochlorothiazide. In the case of Tablet strengths 20/12.5 of lisinopril and hydrochlorothiazide, dilute with water to obtain a concentration of lisinopril and hydrochlorothiazide of 0.4/0.25 mg per mL.

Assay preparation— Dilute the Assay stock preparation quantitatively with Buffer solution to obtain a solution having a final concentration of about 0.1 mg per mL of lisinopril and 0.12 mg per mL of hydrochlorothiazide for Tablet strengths of 10/12.5 and 20/25 of lisinopril and hydrochlorothiazide. In the case of Tablet strengths 20/12.5 of lisinopril and hydrochlorothiazide, the concentration of lisinopril and hydrochlorothiazide is 0.2/0.125 mg per mL. Pass a portion of this solution through a filter having a 0.45-µm or finer porosity, and use the filtrate.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 215-nm detector and a 4.6-mm × 15-cm column that contains 5-µm packing L7. The flow rate is about 1.5 mL per minute, and the run time is about 10 minutes. Chromatograph the Combined standard preparation, and record the peak responses as directed for Procedure: the resolution, R, between lisinopril and benzothiadiazine related compound A is greater than 3.0 and between hydrochlorothiazide and benzothiadiazine related compound A is greater than 4.0; the tailing factor for lisinopril and hydrochlorothiazide peaks is less than 2; and the relative standard deviation for replicate injections is not more than 2.0% for lisinopril and hydrochlorothiazide peaks.

Procedure— Separately inject equal volumes (about 10 µL) of the Combined standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the percentage of lisinopril (C21H31N3O5) and hydrochlorothiazide (C7H8ClN3O4S2) based on the label claim, in the portion of Tablets taken by the formula: in which CS is the concentration, in mg per mL, of USP Lisinopril RS or USP Hydrochlorothiazide RS in the Combined standard preparation; CU is the concentration, in mg per mL, of lisinopril or hydrochlorothiazide in the Assay preparation based on the label claim; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.USP32