A: It meets the requirements under Thin-Layer Chromatographic Identification Test á201BNPñ.

B: The retention time of the major peak for lidocaine in the chromatogram of theAssay preparationcorresponds to that in the chromatogram of theStandard preparation,as obtained in theAssay for lidocaine.

Mobile phase— Dissolve 4.44g of docusate sodium in 1000mLof a mixture of methanol and water (4:1),add 1mLof 0.1Nsulfuric acid,and mix.Make adjustments if necessary (seeSystem SuitabilityunderChromatography á621ñ).

Standard preparation— Dissolve a suitable quantity of USP Lidocaine RS,accurately weighed,inMobile phase to obtain a solution having a known concentration of about 0.4mg per mL.

Assay preparation— Transfer an accurately weighed quantity of Ointment,equivalent to about 40mg of lidocaine,to a separator,add 50mLofn-hexane,and shake until the specimen is in solution.Add 30mLofMobile phase,shake for 1minute,and allow the layers to separate.Drain the lower layer into a 100-mLvolumetric flask,and extract then-hexane layer remaining in the separator with two 30-mLportions ofMobile phase,combining the lower layers in the volumetric flask.Dilute the combined extracts in the 100-mLvolumetric flask withMobile phase to volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 230-nm detector and a 4-mm ×25-cm column that contains packing L1.The flow rate is about 1mLper minute.Chromatograph theStandard preparation,and record the peak response as directed underProcedure:the column efficiency determined from the analyte peak is not less than 500theoretical plates,and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20µL)of theStandard preparation and theAssay preparation into the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of lidocaine (C14H22N2O)in the portion of Ointment taken by the formula: in whichCis the concentration,in mg per mL,of USP Lidocaine RSin theStandard preparation,andrUandrSare the peak responses obtained from theAssay preparation and theStandard preparation,respectively.