(Official January 1, 2007)

pH 6.8 buffer— Dissolve 0.54 g of monobasic potassium phosphate in 3600 mL of water, adjust with 0.5 N sodium hydroxide or 0.5 M phosphoric acid to a pH of 6.8 ± 0.1, dilute with water to make 4000 mL of solution, and mix. Filter this solution through a filter of 0.5 µm or finer porosity.

Mobile phase— Dissolve 2.0 g of sodium 1-hexanesulfonate in 800 mL of pH 6.8 buffer, adjust with 0.5 N sodium hydroxide or 0.5 M phosphoric acid to a pH of 6.8 ± 0.1, and dilute with pH 6.8 buffer to make 1000 mL of solution. Filter this solution through a filter of 0.5 µm or finer porosity, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Imipenem standard preparation— Transfer about 13 mg of USP Imipenem Monohydrate RS, accurately weighed, to a 25-mL volumetric flask. Add 5 mL of saline TS, 0.5 mL of a 0.1% solution of sodium bicarbonate, and about 15 mL of pH 6.8 buffer, and dissolve by shaking and sonicating. [NOTE—The duration of sonication should not exceed 1 minute.] Dilute with pH 6.8 buffer to volume, and mix. This solution contains the equivalent of about 500 µg of anhydrous imipenem per mL. Use this solution immediately.

Cilastatin standard preparation— Transfer about 12.5 mg of USP Cilastatin Ammonium Salt RS, accurately weighed, to a 25-mL volumetric flask. Add 5 mL of saline TS, 0.5 mL of a 0.1% solution of sodium bicarbonate, and about 15 mL of pH 6.8 buffer, and dissolve by shaking and sonicating. [NOTE—The duration of sonication should not exceed 1 minute.] Dilute with pH 6.8 buffer to volume, and mix. This solution contains the equivalent of about 500 µg of cilastatin per mL. Use this solution immediately.

Assay preparation— Constitute Imipenem and Cilastatin for Injection in a volume of saline TS, accurately measured, corresponding to the volume of solvent specified in the labeling. Quantitatively transfer this suspension to a 100-mL volumetric flask with the aid of pH 6.8 buffer, dilute with pH 6.8 buffer to volume, and mix. Dilute an accurately measured volume of this solution quantitatively with pH 6.8 buffer to obtain an Assay preparation having a concentration of about 500 µg of imipenem per mL.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 30-cm column that contains packing L1, and is maintained at a temperature of 50 ± 1.0. The flow rate is about 2 mL per minute. Chromatograph the Imipenem standard preparation, and record the peak responses as directed under Procedure: the column efficiency determined from the imipenem peak is not less than 600 theoretical plates when calculated by the formula: the tailing factor for the imipenem peak is not more than 1.5 when calculated by the formula: where W0.1 is the width of the peak at 10% height, and the relative standard deviation for replicate injections is not more than 2.0%. Chromatograph the Cilastatin standard preparation, and record the peak responses as directed under Procedure: the column efficiency determined from the cilastatin peak is not less than 600 theoretical plates when calculated by the formula: the tailing factor for the cilastatin peak is not more than 1.5 when calculated by the formula: and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Imipenem standard preparation, the Cilastatin standard preparation, and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantities, in mg, of anhydrous imipenem (C12H17N3O4S) and cilastatin (C16H26N2O5S) in the container, taken by the same formula: in which C is the concentration, in mg per mL, of USP Imipenem Monohydrate RS or USP Cilastatin Ammonium Salt RS in the appropriate Standard preparation; P is the content, in µg per mg, of anhydrous imipenem (C12H17N3O4S) or cilastatin (C16H26N2O5S) in the relevant Reference Standard; L is the labeled quantity, in mg, of imipenem or cilastatin in the container; D is the concentration, in µg per mL, of imipenem or cilastatin in the Assay preparation based on the labeled quantity in the container and the extent of dilution; and rU and rS are the peak responses of the corresponding analyte obtained from the Assay preparation and the appropriate Standard preparation, respectively.