Mobile phaseand Chromatographic system— Proceed as directed in the Assayunder Dibucaine.

Standard preparation— Transfer about 25mg of USP Dibucaine Hydrochloride RS,accurately weighed,to a 100-mLvolumetric flask,add 20.0mLof 0.1Nhydrochloric acid,and swirl to dissolve.Dilute with methanol to volume,and mix.Pass through a suitable filter of 0.5-µm or finer porosity.

Assay preparation— Weigh accurately a portion of Cream,equivalent to about 22mg of dibucaine,transfer to a separator containing 25mLof ether,and mix to dissolve.Extract successively with two 9-mLportions of 0.1Nhydrochloric acid,combining the extracts in a 100-mLvolumetric flask.Extract the ether phase in the separator with 2mLof water,collecting the aqueous extract in the 100-mLvolumetric flask.Dilute with methanol to volume,and mix.Pass through a suitable filter of 0.5-µm or finer porosity.

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the area responses for the major peaks.Calculate the quantity,in mg,of dibucaine (C20H29N3O2)in the portion of Cream taken by the formula: in which 343.46and 379.93are the molecular weights of dibucaine and dibucaine hydrochloride,respectively;Cis the concentration,in mg per mL,of USP Dibucaine Hydrochloride RSin the Standard preparation;and rUand rSare the area responses of the dibucaine peaks obtained from the Assay preparationand the Standard preparation,respectively.