A:Infrared Absorption á197Mñ.

B: The retention time of the major peak in the chromatogram of the Assay preparationcorresponds to that in the chromatogram of the Standard preparation,as obtained in the Assay.

Test solution: 10mg per mL,in methanol.

pH6.8Phosphate buffer,pH2.5Phosphate buffer,Solution A,Solution B,Mobile phase,Diluent,Enalapril diketopiperazine solution,System suitability solution,and Chromatographic system— Proceed as directed in the Assay.

Standard solution— Dissolve an accurately weighed quantity of USP Enalapril Maleate RSin Diluent,and dilute quantitatively,and stepwise if necessary,with Diluentto obtain a solution having a known concentration of about 3µg per mL.

Test solution— Use the Assay preparation.

Procedure— Separately inject equal volumes (about 50µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the peak area responses.Calculate the percentage of each impurity in the portion of Enalapril Maleate taken by the formula: in which CSis the concentration,in mg per mL,of USP Enalapril Maleate RSin the Standard solution;CTis the concentration,in mg per mL,of Enalapril Maleate in the Test solution;riis the peak area of each impurity obtained from the Test solution;and rSis the peak area of enalapril obtained from the Standard solution:not more than 1.0%of any impurity having a relative retention time of about 1.10is found;not more than 0.3%of any other individual impurity is found;and not more than 2%of total impurities is found.

pH6.8Phosphate buffer— Dissolve 2.8g of monobasic sodium phosphate in about 900mLof water in a 1000-mLvolumetric flask.Adjust with a 9Msodium hydroxide solution to a pHof about 6.8,dilute with water to volume,and mix.

pH2.5Phosphate buffer— Dissolve 2.8g of monobasic sodium phosphate in about 900mLof water in a 1000-mLvolumetric flask.Adjust with phosphoric acid to a pHof about 2.5,dilute with water to volume,and mix.

Solution A— Prepare a filtered and degassed mixture of pH6.8Phosphate bufferand acetonitrile (19:1).

Solution B— Prepare a filtered and degassed mixture of acetonitrile and pH6.8Phosphate buffer(33:17).

Mobile phase— Use variable mixtures of Solution Aand Solution Bas directed for Chromatographic system.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Diluent— Prepare a mixture of pH2.5Phosphate bufferand acetonitrile (95:5).

Enalapril diketopiperazine solution— Carefully place about 20mg of USP Enalapril Maleate RSin a 100-mLbeaker 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.Heat for about 5to 10minutes until the solid is melted.Immediately remove the beaker from the hot plate,and allow to cool.[NOTE—Avoid overheating to prevent heat-induced degradation,which would give rise to a brown color.]To the cooled residue in the beaker add 50mLof acetonitrile,and sonicate for a few minutes to dissolve.The solution typically contains,in each mL,between 0.2mg and 0.4mg of enalapril diketopiperazine.

Standard preparation— Dissolve an accurately weighed quantity of USP Enalapril Maleate RSin Diluent,and dilute quantitatively,and stepwise if necessary,with Diluentto obtain a solution having a known concentration of about 0.3mg per mL.

System suitability solution— Add 1mLof Enalapril diketopiperazine solutionto a 50-mLportion of the Standard preparation,and mix.

Assay preparation— Transfer about 30mg of Enalapril Maleate,accurately weighed,to a 100-mLvolumetric flask,dissolve in and dilute with Diluentto volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 215-nm detector and a 4.1-mm ×15-cm column that contains packing L21.The flow rate is about 1.5mLper minute.The column temperature is maintained at 70.The chromatograph is programmed as follows. Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 1.0for enalapril and 2.1for enalapril diketopiperazine;and the resolution,R,between enalapril and enalapril diketopiperazine is not less than 3.5.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 50µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C20H28N2O5·C4H4O4in the portion of Enalapril Maleate taken by the formula: in which Cis the concentration,in mg per mL,of USP Enalapril Maleate RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.