A: Infrared Absorption á197Kñ.

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

1000(C/W)(rE/rS),

Potassium phosphate buffer ,Mobile phase,System suitability solution,Solvent,and Chromatographic system—Proceed as directed in the Assay.

Standard solution— Quantitatively dissolve an accurately weighed quantity of USP Dirithromycin RSin Solventto obtain a solution having a known concentration of about 0.2mg per mL.

Test solution— Transfer about 100mg of Dirithromycin,accurately weighed,to a 10-mLvolumetric flask,dissolve in and dilute with Solventto volume,and mix.

Procedure— [NOTE—Use peak areas where peak responses are indicated.]Separately inject equal volumes (about 10µL)of the Standard solutionand the Test solutioninto the chromatograph,and record the chromatograms for a period of time that is not less than three times the retention time of dirithromycin (16R-epimer).Calculate the percentage of each impurity found in the portion of Dirithromycin taken by the formula: in which Cis the concentration,in mg per mL,of USP Dirithromycin RSin the Standard solution;Wis the quantity,in mg,of Dirithromycin taken to prepare the Test solution;riis the response for each impurity found in the chromatogram of the Test solution;and rSis the response of the dirithromycin (16R-epimer)in the chromatogram of the Standard solution:not more than 1.5%of 9-(S)-erythromycylamine is found;not more than 1.0%of any other individual impurity is found;and not more than 4.0%of total impurities is found.[NOTE—Do not regard dirithromycin 16S-epimer as an impurity.]

Potassium phosphate buffer— Dissolve 1.41g of monobasic potassium phosphate and 6.91g of dibasic potassium phosphate in 1liter of water,and pass through a filter having a porosity of 0.5µm or finer.

Mobile phase— Prepare a degassed mixture of acetonitrile,Potassium phosphate buffer,and methanol (44:37:19).

System suitability solution— Dissolve an accurately weighed quantity of USP Dirithromycin RSin Mobile phaseto obtain a solution having a concentration of about 2.5mg per mL.Store this solution at room temperature for about 24hours.[NOTE—The solution then contains an equilibrated mixture of dirithromycin (16R-epimer),dirithromycin 16S-epimer,and 9-(S)-erythromycylamine.The solution may be used for 1month when stored at room temperature.]

Solvent— Prepare a mixture of acetonitrile and methanol (70:30).

Standard preparation— Quantitatively dissolve an accurately weighed quantity of USP Dirithromycin RSin Solventto obtain a solution having a known concentration of about 2mg per mL.

Assay preparation— Transfer about 20mg of Dirithromycin,accurately weighed,to a 10-mLvolumetric flask,dissolve in and dilute with Solventto volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 205-nm detector and a 4.6-mm ×25-cm column that contains 5-µm packing L1and is maintained at a constant temperature of about 40.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 0.7for 9-(S)-erythromycylamine,1.0for dirithromycin (16R-epimer),and 1.12for dirithromycin 16S-epimer.The resolution,R,between dirithromycin (16R-epimer)and dirithromycin 16S-epimer is not less than 2.0,and between dirithromycin (16R-epimer)and 9-(S)-erythromycylamine is not less than 5.0,the tailing factor for the dirithromycin (16R-epimer)peak is not more than 2.0,and the relative standard deviation of the dirithromycin (16R-epimer)peak 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 areas of the major peaks.Calculate the percentage of C42H78N2O14in the portion of Dirithromycin taken by the formula: in which Cis the concentration,in mg per mL,of USP Dirithromycin RSin the Standard preparation;Wis the quantity,in mg,of Dirithromycin taken to prepare the Assay preparation;rUand rSare the area responses for dirithromycin (16R-epimer)obtained from the Assay preparationand the Standard preparation,respectively;and PEis the percentage of 16S-epimer as determined in the test for Limit of dirithromycin 16S-epimer.