Medium: pH6.8phosphate buffer (see Buffer Solutionsin the section Reagents,Indicators,and Solutions);900mL.

Apparatus 2: 50rpm.

Time: 30minutes.

Procedure— Determine the amount of C20H28N2O5·C4H4O4dissolved as directed for Procedure for content uniformityunder Uniformity of dosage unitsexcept to use pH6.8phosphate buffer instead of the Buffer solutionto prepare the Standard preparation,to use a filtered portion of the solution under test as theTest 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·C4H4O4is dissolved in 30minutes.

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 1.0mLof the Standard preparationto a 100-mLvolumetric flask,dilute with Buffer solutionto volume,and mix.

Procedure— Separately inject equal volumes (about 50µL)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 preparationgreater 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 492.52and 348.39are the molecular weights of enalapril maleate and anhydrous enalaprilat,respectively;Cis the concentration,in mg per mL,of USP Enalaprilat RSin theStandard preparation;Vis the nominal capacity,in mL,of the volumetric flask containing the Test preparation;Nis the number of Tablets taken for the Test preparation;rUand rSare the enalaprilat peak responses obtained from the Test preparationand the Standard preparation,respectively;and Lis the labeled amount of enalapril maleate in the Tablet.

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

Mobile phase— Prepare a filtered and degassed mixture of Buffer solutionand acetonitrile (75:25).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

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

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 to melt the solid.When melting is observed,(after 5to 10minutes 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 50mLof acetonitrile and sonicate for a few minutes to dissolve the residue.The solution typically contains,in each mL,between 0.2mg and 0.4mg of enalapril diketopiperazine.

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

System suitability solution— Transfer 0.5mLof Enalapril diketopiperazine solutionto a 25-mLvolumetric flask,dilute with Standard preparationto volume,and mix.

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

Chromatographic system (see Chromatography á621ñ)— 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 2mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.3for maleic acid,0.5for enalaprilat,1.0for enalapril,and 1.5for enalapril diketopiperazine.[NOTE—Apeak 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 1000theoretical plates for enalaprilat,not less than 300theoretical plates for enalapril,and not less than 2500theoretical 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 preparationas 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 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 enalapril maleate (C20H28N2O5·C4H4O4)in each Tablet taken by the formula: in which Cis the concentration,in mg per mL,of USP Enalapril Maleate RSin the Standard preparation;Vis the nominal capacity,in mL,of the volumetric flask containing the Assay preparation;Nis the number of Tablets taken for the Assay preparation;and rUand rSare the enalapril peak responses obtained from the Assay preparationand the Standard preparation,respectively.