Medium: pH 6.8 phosphate buffer (see Buffer Solutions in the section Reagents, Indicators, and Solutions); 900 mL.

Apparatus 2: 50 rpm.

Time: 30 minutes.

Procedure— Determine the amount of C20H28N2O5·C4H4O4 dissolved as directed for Procedure for content uniformity under Uniformity of dosage units except to use pH 6.8 phosphate buffer instead of the Buffer solution to prepare the Standard preparation, to use a filtered portion of the solution under test as the Test preparation, and to make any necessary modifications for appropriate sample and standard concentrations.

Tolerances— Not less than 80% (Q) of the labeled amount of C20H28N2O5·C4H4O4 is dissolved in 30 minutes.

Procedure for content uniformity—

Buffer solution, Mobile phase, Enalaprilat standard solution, Enalapril diketopiperazine solution, System suitability solution, Standard preparation, and Chromatographic system— Proceed as directed in the Assay.

Test preparation— Use the Assay preparation.

Related compounds standard solution— Transfer 1.0 mL of the Standard preparation to a 100-mL volumetric flask, dilute with Buffer solution to volume, and mix.

Procedure— Separately inject equal volumes (about 50 µL) of the Standard preparation, the Test preparation, the Related compounds standard solution, and the Buffer solution, into the chromatograph, record the chromatograms, and measure the responses for all of the peaks in the Test preparation greater than 0.1% of the response of the enalapril peak that are not observed in the Buffer solution. Calculate the percentage of anhydrous enalaprilat (as enalapril maleate) present in the portion of Tablets taken by the formula: in which 492.52 and 348.39 are the molecular weights of enalapril maleate and anhydrous enalaprilat, respectively; C is the concentration, in mg per mL, of USP Enalaprilat RS in the Standard preparation; V is the nominal capacity, in mL, of the volumetric flask containing the Test preparation; N is the number of Tablets taken for the Test preparation; rU and rS are the enalaprilat peak responses obtained from the Test preparation and the Standard preparation, respectively; and L is the labeled amount of enalapril maleate in the Tablet.

(Official January 1, 2007)

Buffer solution— Dissolve 1.38 g of monobasic sodium phosphate in about 800 mL of water. Adjust with phosphoric acid to a pH of 2.2, dilute with water to 1000 mL, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Buffer solution and acetonitrile (75:25). Make adjustments if necessary (see System Suitability under Chromatography 621).

Enalaprilat standard solution— Dissolve an accurately weighed quantity of USP Enalaprilat RS in water to obtain a solution having a known concentration of about 0.4 mg per mL.

Enalapril diketopiperazine solution— Carefully place about 20 mg of USP Enalapril Maleate RSin a 100-mL beaker to form a mound on the bottom of the beaker. Place the beaker on a hot plate at about one-half the maximum hot plate temperature setting to melt the solid. When melting is observed, (after 5 to 10 minutes of heating), immediately remove the beaker from the hot plate, and allow it to cool. [NOTE—Avoid over heating beyond the melting initially observed to prevent heat-induced degradation, which would give rise to a brown color.] To the cooled residue in the beaker add 50 mL of acetonitrile and sonicate for a few minutes to dissolve the residue. The solution typically contains, in each mL, between 0.2 mg and 0.4 mg of enalapril diketopiperazine.

Standard preparation— Transfer about 20 mg of USP Enalapril Maleate RS, accurately weighed, to a 100-mL volumetric flask. Transfer 0.5 mL of Enalaprilat standard solution to the flask, and add about 50 mL of Buffer solution to dissolve, using sonication if necessary. Dilute with Buffer solution to volume, and mix to obtain a solution having known concentrations of about 0.2 mg of USP Enalapril Maleate RS per mL and 0.002 mg of USP Enalaprilat RS per mL.

System suitability solution— Transfer 0.5 mL of Enalapril diketopiperazine solution to a 25-mL volumetric flask, dilute with Standard preparation to volume, and mix.

Assay preparation— Transfer not fewer than 10 Tablets to a volumetric flask of capacity such that, when filled to volume, will produce a solution having a concentration of about 0.2 mg of enalapril maleate per mL. Add a volume of Buffer solution that is about one-half the nominal volume of the flask, sonicate for 15 minutes, and shake by mechanical means for 30 minutes. Dilute with Buffer solution to volume, shake well, and sonicate for 15 minutes. Pass through a filter of 0.45-µm or finer porosity, discarding the first portion of the filtrate.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 215-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L7. The column temperature is maintained at 50, and the flow rate is about 2 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.3 for maleic acid, 0.5 for enalaprilat, 1.0 for enalapril, and 1.5 for enalapril diketopiperazine. [NOTE—A peak response for heat-induced degradation product of enalapril diketopiperazine (if present with relative retention time about 1.2) is not greater than 15% of the response for enalapril diketopiperazine.] The column efficiency is not less than 1000 theoretical plates for enalaprilat, not less than 300 theoretical plates for enalapril, and not less than 2500 theoretical plates for enalapril diketopiperazine; the tailing factor for enalapril is not more than 2.0; the resolution, R, between maleic acid and enalaprilat is not less than 2.0, between enalaprilat and enalapril is not less than 2.0, and between enalapril and enalapril diketopiperazine is not less than 2.0. Chromatograph the Standard preparation as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0% for the enalapril peak, and responses for the enalaprilat peak agree within 5%.

Procedure— Separately inject equal volumes (about 50 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 enalapril maleate (C20H28N2O5·C4H4O4) in each Tablet taken by the formula: in which C is the concentration, in mg per mL, of USP Enalapril Maleate RS in the Standard preparation; V is the nominal capacity, in mL, of the volumetric flask containing the Assay preparation; N is the number of Tablets taken for the Assay preparation; and rU and rS are the enalapril peak responses obtained from the Assay preparation and the Standard preparation, respectively.