Packaging and storage— Preserve in well-closed,light-resistant containers.

USP Reference standards á11ñ— USP Bacitracin Zinc RS.USP Polymyxin B Sulfate RS.

Thin-layer chromatographic identification test á201BNPñ: meets the requirements.

Minimum fill á755ñ: meets the requirements.

Water,Method Iá921ñ: not more than 0.5%,20mLof a mixture of toluene and methanol (7:3)being used in place of methanol in the titration vessel.

Assay for bacitracin— Proceed with Ointment as directed in the Assayunder Bacitracin Zinc Ointment.

Assay for polymyxin B— Proceed as directed for polymyxin Bunder Antibiotics—Microbial Assays á81ñ,using an accurately weighed portion of Ointment shaken with about 50mLof ether in a separator,and extracted with four 20-mLportions of Buffer No.6.Combine the aqueous extracts,and dilute with Buffer No.6to an appropriate volume to obtain a stock solution.Dilute this stock solution quantitatively and stepwise with Buffer No.6to obtain a Test Dilutionhaving a concentration assumed to be equal to the median dose level of the Standard.

Expert Committee:(PA7)Pharmaceutical Analysis 7

USP28–NF23Page 216

Phone Number:1-301-816-8335