0.1 M Ammonium acetate, Solution A, Solution B, Mobile phase, pH 6.3 Phosphate buffer, and Chromatographic system— Proceed as directed in the Assay.

Sodium acetate solution— Add 5.8 mL of glacial acetic acid to 900 mL of water, adjust with sodium hydroxide TS to a pH of 4.0, dilute with water to 1000 mL, and mix.

Tetrahydrofuran solution— Mix 750 mL of tetrahydrofuran and 250 mL of water.

Sodium acetate and tetrahydrofuran solution— Prepare a mixture of Sodium acetate solution and Tetrahydrofuran solution (50:50).

Standard solution— Dissolve an accurately weighed portion of USP Mupirocin Lithium RS in pH 6.3 Phosphate buffer. Dilute an accurately measured volume of this solution quantitatively to obtain a solution containing 0.1 mg of mupirocin per mL.

Test stock solution —Transfer an accurately weighed quantity of Cream, equivalent to about 50 mg of mupirocin, to a screw-capped centrifuge tube. Add 5.0 mL of Tetrahydrofuran solution, cap, and disperse the Cream by mixing on a vortex mixer and shaking. Add 5.0 mL of Sodium acetate solution, cap, and mix. Centrifuge for about 15 minutes. Withdraw the lower layer from the tube, pass it through a filter having a 0.5-µm or finer porosity, and use the filtrate.

Test solution— Transfer 1.0 mL of the Test stock solution to a 50-mL volumetric flask, dilute with Sodium acetate and tetrahydrofuran solution to volume, mix, and pass through a filter having a 0.5-µm or finer porosity.

pH 4 Acetate buffer— Transfer about 13.6 g of sodium acetate to a 1000-mL volumetric flask, and dissolve in about 900 mL of water. Adjust with glacial acetic acid to a pH of 4.0, and dilute with water to volume.

Chromatographic system (see Chromatography 621)— Chromatograph the Standard solution, and record the peak responses as directed for Procedure: typical retention times are about 16 minutes for pseudomonic acid D and 21 minutes for mupirocin; the relative retention times are 0.36 for pseudomonic acid F, 0.6 for mupirocin degradation product A, 0.63 for mupirocin degradation product B, 0.74 for pseudomonic acid D, 0.9 for pseudomonic acid B, 1.0 for mupirocin, 1.15 for mupirocin related compound A, 1.23 for mupirocin related compound B, 2.03 for pseudomonic acid C, and 2.15 to 2.33 for pseudomonic acid E; the resolution, R, between pseudomonic acid D and mupirocin is not less than 3; the column efficiency for the mupirocin peak is not less than 7000 theoretical plates; the tailing factor for the mupirocin peak is not more than 1.75; and the relative standard deviation of the mupirocin peak for replicate injections is not more than 2%.

Procedure— [note—Ensure that buffers, dispersants, or preservatives in the formulation do not interfere with quantification of either impurities or degradation products.] Separately inject equal volumes (about 20 µL) of the Test stock solution and the Test solution into the chromatograph, and measure the peak responses for all of the peaks that do not correspond to buffers, dispersants, or preservatives. Calculate the percentage of each related compound and degradation product relative to mupirocin in the portion of Cream taken by the formula: in which ri is the peak response for each related compound or degradation product obtained from the Test stock solution; and rM is the peak response of the mupirocin peak obtained from the Test solution: not more than 3.0% of pseudomonic acid D is found; not more than 8.5% of mupirocin degradation product A is found; not more than 16% of mupirocin degradation product B is found; not more than 1.2% of any other individual impurity or degradation product is found; and not more that 30% of total impurities and degradation products is found.

0.1 M Ammonium acetate— Prepare as directed in the Assay under Mupirocin Calcium.

Solution A— Prepare a filtered and degassed mixture of 0.1 M Ammonium acetate and tetrahydrofuran (75:25).

Solution B— Prepare a filtered and degassed mixture of 0.1 M Ammonium acetate and tetrahydrofuran (70:30).

Mobile phase— Use variable mixtures of Solution A and Solution B as directed for Chromatographic system. Make adjustments if necessary (see System Suitability under Chromatography 621).

pH 6.3 Phosphate buffer— Dissolve 69 g of monobasic sodium phosphate in 800 mL of water, adjust with sodium hydroxide TS to a pH of 6.3, dilute with water to 1000 mL, and mix.

Standard preparation— Transfer about 21 mg of USP Mupirocin Lithium RS, accurately weighed, to a 200-mL volumetric flask, and dissolve in and dilute with pH 6.3 Phosphate buffer to volume.

Assay preparation— Transfer an accurately weighed quantity of Cream, equivalent to about 10 mg of mupirocin, to a 100-mL volumetric flask. Add 50 mL of pH 6.3 Phosphate buffer and 25 mL of tetrahydrofuran. Insert the stopper into the flask, mix on a vortex mixer, and shake for 1 to 3 minutes. Dilute with pH 6.3 Phosphate buffer to volume. Allow to stand until the oil layer separates out, then dilute the aqueous layer with pH 6.3 Phosphate buffer to volume. Repeat 2 to 3 times until as much of the oil layer has separated out as possible. After the final dilution, pass the final solution (bottom layer) through a filter having a 0.5-µm or finer porosity.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 240-nm detector and a 4.6-mm × 25-cm column that contains 7-µm packing L7. The flow rate is about 1 mL per minute. Maintain the column at a constant temperature up to 35. The chromatograph is programmed as follows. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: typical retention times are about 16 minutes for pseudomonic acid D and 21 minutes for mupirocin; the resolution, R, between pseudomonic acid D and mupirocin is not less than 3; the column efficiency for the mupirocin peak is not less than 7000 theoretical plates; the tailing factor for the mupirocin peak is not more than 1.75; and the relative standard deviation of the mupirocin peak for replicate injections is not more than 2%.

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 peak area responses for the major peaks. Calculate the weight percent of mupirocin in the portion of Cream taken by the formula: in which MS is the weight, in mg, of USP Mupirocin Lithium RS taken to prepare the Standard preparation; E is the designated mupirocin equivalent, in µg, of mupirocin in each mg of USP Mupirocin Lithium RS; MU is the weight, in mg, of Cream taken to prepare the Assay preparation; and rU and rS are the mupirocin peak area responses obtained from the Assay preparation and the Standard preparation, respectively.