A: Infrared Absorption 197M.

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

Test solution: 5 mg per mL, in acetonitrile.

Mobile phase, Diluent, and Chromatographic system— Proceed as directed in the Assay.

Test solution— Use the Assay preparation.

Procedure— Inject about 5 µL of the Test solution into the chromatograph, record the chromatogram, and measure the areas for all the peaks. Calculate the percentage of each impurity in the portion of Simvastatin taken by the formula: in which ri is the peak area for each impurity; and rs is the sum of the areas of all the peaks. Not more than 1.0% of the sum of lovastatin and epilovastatin is found. [note—If present, lovastatin and epilovastatin may not be completely resolved by the method. These peaks, appearing at a relative retention time of 0.6, are integrated together to determine conformance.] Not more than 0.4% of any individual impurity other than lovastatin and epilovastatin is found; and not more than 1.0% of total impurities other than lovastatin and epilovastatin is found.

Dilute phosphoric acid— Transfer 1 mL of phosphoric acid to a 1-L volumetric flask, and dilute with water to volume.

Solution A— Prepare a mixture of acetonitrile and Dilute phosphoric acid (50:50).

Solution B— Transfer 1 mL of phosphoric acid to a 1-L volumetric flask, and dilute with acetonitrile to volume.

Mobile phase— Use variable mixtures of Solution A and Solution B, as directed for Chromatographic system. Make adjustments if necessary (see System Suitability under Chromatography 621).

Buffer solution— Prepare a solution containing 1.4 g of monobasic potassium phosphate per L, and adjust with phosphoric acid to a pH of 4.0.

Diluent— Prepare a mixture of acetonitrile and Buffer solution (3:2).

System suitability preparation— Dissolve accurately weighed quantities of USP Simvastatin RS and USP Lovastatin RS in Diluent, and dilute quantitatively, and stepwise if necessary, with Diluent to obtain a solution having known concentrations of about 1.5 mg per mL of USP Simvastatin RS and 0.015 mg per mL of USP Lovastatin RS.

Standard preparation— Dissolve an accurately weighed quantity of USP Simvastatin RS in Diluent to obtain a solution having a known concentration of about 1.5 mg per mL.

Assay preparation— Transfer about 75 mg of Simvastatin, accurately weighed, to a 50-mL volumetric flask, dissolve in and dilute with Diluent to volume, and mix.

Chromatographic system (see Chromatography 621— The liquid chromatograph is equipped with a 238-nm detector and a 4.6- × 33-mm column that contains packing L1. The flow rate is about 3.0 mL per minute. The chromatograph is programmed as follows. Chromatograph the System suitability preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.60 for lovastatin and 1.0 for simvastatin; and the resolution, R, between simvastatin and lovastatin is greater than 3. 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 5 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the quantity, in mg, of C25H38O5 in the portion of Simvastatin taken by the formula: in which V is the volume, in mL, of the Assay preparation; C is the concentration, in mg per mL, of USP Simvastatin RS in the Standard preparation; and rU and rS are the responses of the simvastatin peak obtained from the Assay preparation and the Standard preparation, respectively.