» Metronidazole Gel contains not less than 90.0 percent and not more than 110.0 percent of the labeled amount of metronidazole (C6H9N3O3).
Packaging and storage Preserve in laminated collapsible tubes at controlled room temperature.
USP Reference standards 11
A: Thin-Layer Chromatographic Identification Test 201
Test solution Transfer a weighed quantity of the Gel, equivalent to 7.5 mg of metronidazole, to a suitable flask, add 15 mL of water, shake to disperse, and sonicate for about 10 minutes. Elute a portion of this solution through a 10-mm × 15-cm chromatographic column containing a 10-cm length of ion-exchange resin with a pledget of glass wool at the bottom and top of the resin, and collect the eluate in a suitable vial.
Standard solution: 0.5 mg per mL.
Application volume: 5 µL.
Developing solvent system: a mixture of chloroform, methanol, and ammonium hydroxide (6:3:1).
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.
Minimum fill 755: meets the requirements.
pH 791 The apparent pH, determined potentiometrically, is between 4.0 and 6.5.
Mobile phase Dissolve 1.5 g of monobasic potassium phosphate and 1.3 g of dibasic sodium phosphate in 350 mL of water, add 650 mL of methanol, mix, filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).
Standard preparation Dissolve an accurately weighed quantity of USP Metronidazole RS in Mobile phase, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having a known concentration of about 0.075 mg per mL.
Assay preparation Transfer an accurately weighed quantity of Gel, equivalent to 7.5 mg of metronidazole, to a 100-mL volumetric flask, add 50 mL of Mobile phase, and shake by mechanical means for 20 minutes. Dilute with Mobile phase to volume, and mix. Centrifuge a portion of this solution until clear, and use the supernatant for injection into the chromatograph.
Chromatographic system (see Chromatography 621)The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains 5-µm packing L7. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak heights as directed for Procedure: the tailing factor for the metronidazole peak is not more than 2.0; and the relative standard deviation for replicate injections is not more than 2.0%.
Procedure Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the heights of the major peaks. Calculate the quantity, in mg, of metronidazole (C6H9N3O3) in the portion of Gel taken by the formula:
100C(rU / rS)in which C is the concentration, in mg per mL, of USP Metronidazole RS in the Standard preparation; and rU and rS are the peak heights of metronidazole obtained from the Assay preparation and the Standard preparation, respectively.
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