A: Infrared Absorption 197K.

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.

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 RS in Solvent to obtain a solution having a known concentration of about 0.2 mg per mL.

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

Procedure— [NOTE—Use peak areas where peak responses are indicated.] Separately inject equal volumes (about 10 µL) of the Standard solution and the Test solution into 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 C is the concentration, in mg per mL, of USP Dirithromycin RS in the Standard solution; W is the quantity, in mg, of Dirithromycin taken to prepare the Test solution; ri is the response for each impurity found in the chromatogram of the Test solution; and rS is 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.]

(Official January 1, 2007)

Potassium phosphate buffer— Dissolve 1.41 g of monobasic potassium phosphate and 6.91 g of dibasic potassium phosphate in 1 L of water, and pass through a filter having a porosity of 0.5 µm 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 RS in Mobile phase to obtain a solution having a concentration of about 2.5 mg per mL. Store this solution at room temperature for about 24 hours. [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 1 month 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 RS in Solvent to obtain a solution having a known concentration of about 2 mg per mL.

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

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 205-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L1 and is maintained at a constant temperature of about 40. The flow rate is about 2 mL per minute. Chromatograph the System suitability solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.7 for 9-(S)-erythromycylamine, 1.0 for dirithromycin (16R-epimer), and 1.12 for 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 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas of the major peaks. Calculate the percentage of C42H78N2O14 in the portion of Dirithromycin taken by the formula: in which C is the concentration, in mg per mL, of USP Dirithromycin RS in the Standard preparation; W is the quantity, in mg, of Dirithromycin taken to prepare the Assay preparation; rU and rS are the area responses for dirithromycin (16R-epimer) obtained from the Assay preparation and the Standard preparation, respectively; and PE is the percentage of 16S-epimer as determined in the test for Limit of dirithromycin 16S-epimer.