Test solution: 5mg per mL,in a pH7buffer.Prepare the pH7buffer solution as follows.Dissolve 5g of monobasic potassium phosphate and 11g of dibasic potassium phosphate in 900mLof water,adjust with phosphoric acid or 5Nsodium hydroxide to a pHof 7,dilute with water to 1000mL,and mix.

Internal standard solution— Add 1mLof n-propyl alcohol to 2000mLof water,and mix.

Standard preparation— Transfer 1.0mLof acetone and 2.0mLof isopropyl alcohol to a 1000-mLvolumetric flask,dilute with water to volume,and mix.Transfer 1.0mLof this solution and 5.0mLof Internal standard solutionto a 25-mLvolumetric flask,dilute with water to volume,and mix.Each mLof this Standard preparationcontains 31.6µg of acetone and 63.2µg of isopropyl alcohol.

Test preparation— Transfer about 250mg of Imipenem,accurately weighed,to a 10-mLvolumetric flask,add 4.0mLof 1Nammonium hydroxide,and dissolve by swirling.Add 2.0mLof Internal standard solution,dilute with water to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The gas chromatograph is equipped with a flame-ionization detector and a 3-mm ×1.8-m column containing 10%phase G16on support S5.The column temperature is programmed to operate at 70for 8minutes,then to increase at a rate of 32per minute to 170,and to maintain the temperature at 170for 8minutes.The injection port is maintained at 200,the detector is maintained at 250,and helium is used as the carrier gas at a flow rate of about 19mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed under Procedure:the relative retention times are about 0.3for acetone,0.5for isopropyl alcohol,and 1.0for n-propyl alcohol,and the relative standard deviation of each of the ratios of the response of the respective analyte peak to the response of the n-propyl alcohol peak for replicate injections is not more than 5%.

Procedure— [NOTE—Use peak areas where peak responses are indicated.]Separately inject equal volumes (about 2µL)of the Standard preparationand the Test preparationinto the chromatograph,using the solvent (water)flush technique,record the chromatograms,and measure the responses for the acetone,isopropyl alcohol,and n-propyl alcohol peaks.Calculate the percentages of acetone and isopropyl alcohol in the portion of Imipenem taken by the same formula: in which Cis the concentration,in µg per mL,of the appropriate analyte in the Standard preparation,Wis the quantity,in mg,of Imipenem taken to prepare the Test preparation,and RUand RSare the ratios of the peak response of each of the corresponding analytes to the peak responses of n-propyl alcohol obtained from the Test preparationand the Standard preparation,respectively.Add the percentages of acetone and isopropyl alcohol found:the total is not more than 0.25%.

Mobile phase— Dissolve 0.54g of monobasic potassium phosphate in 3600mLof water,adjust with 0.5Nsodium hydroxide or 0.5Mphosphoric acid to a pHof 6.8±0.1,dilute with water to make 4000mLof solution,and mix.Filter this solution through a filter of 0.5-µm or finer porosity,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Imipenem Monohydrate RSin Mobile phaseto obtain a solution having a known concentration of about 0.4mg per mL.Store this solution in an ice bath,and discard after 8hours.

Assay preparation— Transfer about 100mg of Imipenem,accurately weighed,to a 250-mLvolumetric flask,dissolve in and dilute with Mobile phaseto volume,and mix.Store this solution in an ice bath,and discard the unused portion after 8hours.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 300-nm detector and a 4.6-mm ×30-cm column that contains packing L1,and is maintained at a temperature of 30±1.0.The flow rate is about 1.5mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed under Procedure:the column efficiency determined from the analyte peak is not less than 600theoretical plates,and the relative standard deviation for replicate injections is not more than 1.0%.

Procedure— Separately inject equal volumes (about 10µL)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 imipenem monohydrate (C12H17N3O4S·H2O)in the portion of Imipenem taken by the formula: in which 317.36and 299.35are the molecular weights of imipenem monohydrate and anhydrous imipenem,respectively,Cis the concentration,in mg per mL,of USP Imipenem Monohydrate RSin the Standard preparation,Pis the content,in µg per mg,of anhydrous imipenem (C12H17N3O4S)in USP Imipenem Monohydrate RS,and rUand rSare the imipenem peak responses obtained from the Assay preparationand the Standard preparation,respectively.