pH6.3phosphate buffer— Prepare 0.05Mmonobasic sodium phosphate,and adjust with 10Nsodium hydroxide to a pHof 6.3±0.2.

Mobile phase— Prepare a suitable mixture of pH6.3phosphate bufferand acetonitrile (750:250),pass through a suitable filter of 0.5µm or finer porosity,and degas.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Transfer about 11mg of USP Mupirocin Lithium RS,accurately weighed,to a 100-mLvolumetric flask,add 25mLof acetonitrile,and swirl to dissolve.Dilute with pH6.3phosphate bufferto volume,and mix.

Resolution solution— Adjust 10mLof Standard preparationwith 6Nhydrochloric acid to a pHof 2.0,allow to stand for 2hours,and adjust with 5Nsodium hydroxide to a pHof 6.3±0.2.

Assay preparation— Transfer about 11mg of Mupirocin,accurately weighed,to a 100-mLvolumetric flask,add 25mLof acetonitrile,and swirl to dissolve.Dilute with pH6.3phosphate bufferto volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 229-nm detector and a 4.6-mm ×25-cm column that contains packing L1based on spherical silica particles.The flow rate is about 2mLper minute.Chromatograph the Resolution solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.9for the mupirocin acid hydrolysis product and 1.0for mupirocin,and the resolution,R,between the mupirocin acid hydrolysis product and mupirocin is not less than 2.0.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the tailing factor is not more than 2,the column efficiency is not less than 1500theoretical plates when calculated by the formula: in which the terms are as defined therein.The relative standard deviation for replicate injections is not more than 2.0%.

Procedure— [NOTE—Use peak areas where peak responses are indicated.]Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in µg,of mupirocin (C26H44O9)in each mg of Mupirocin taken by the formula: in which MSis the weight,in mg,of USP Mupirocin Lithium RStaken to prepare the Standard preparation;Eis the mupirocin equivalent,in µg per mg,of USP Mupirocin Lithium RS;MUis the weight,in mg,of mupirocin taken to prepare the Assay preparation;and rUand rSare the mupirocin peak responses obtained from the Assay preparationand the Standard preparation,respectively.