A: Infrared Absorption á197Mñ—[NOTE—If the spectrum is not comparable to that of the Reference Standard,expose the specimen and Reference Standard to an environment of 98%relative humidity (use a chamber conditioned with a saturated solution of calcium sulfate)for 1to 3days to equilibrate them.Prepare dispersions from the equilibrated specimen and Reference Standard,and record the spectra.]

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

Test solution: 10mg per mL,in methanol.

pH3buffer— Dissolve 1.36g of monobasic potassium phosphate in 950mLof water,adjust with phosphoric acid to a pHof 3.0±0.1,dilute with water to 1000mL,and mix.

Solvent mixture— Prepare a mixture of acetonitrile,methanol,and pH3buffer(2:2:1).Adjust with phosphoric acid to a pHof 3.0±0.1,and mix.

Diluent— Prepare a mixture of pH3bufferand Solvent mixture(92:8),and filter.

Mobile phase— Prepare a filtered and degassed mixture of pH3bufferand Solvent mixture(85:15).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Enalaprilat RSin Diluentto obtain a solution having a known concentration of about 0.3mg per mL.[NOTE—Use this solution within 24hours.]

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

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 210-nm detector and a 4.6-mm ×15-cm column that contains 4-µm packing L1and is maintained at 70.The flow rate is about 1.5mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the column efficiency determined from the analyte peak is not less than 500theoretical plates;the tailing factor for the analyte peak is not more than 1.7;and the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 20µL)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 C18H24N2O5in the portion of Enalaprilat taken by the formula: in which Cis the concentration,in mg per mL,of USP Enalaprilat RSin the Standard preparation;and rUand rSare the enalaprilat peak responses obtained from the Assay preparationand the Standard preparation,respectively.