A: The solution employed for measurement of absorbance in the Assay for tetracaineexhibits a maximum at 310±2nm (presence of tetracaine).

B: Dissolve 5g in 50mLof ether,extract the ether solution with 5mLof 3Nhydrochloric acid,and filter the acid extract.Add 2mLof potassium thiocyanate solution (1in 2)to the filtrate:a crystalline precipitate is formed,and when recrystallized from water and dried at 80for 2hours,it melts between 130and 132(see Melting Range or Temperature á741ñ)(presence of tetracaine).

C: When chromatographed as directed in the Assay for menthol,the Assay preparationexhibits a major peak for menthol,the retention time of which corresponds to that exhibited by menthol in the Standard preparation.

Standard preparation— Prepare as directed in the Assayunder Tetracaine Ointment.

Assay preparation— Using Ointment,proceed as directed in the Assayunder Tetracaine Ointment.

Procedure— Proceed as directed for Procedurein the Assayunder Tetracaine Ointment.

Internal standard solution— Dissolve decanol in n-hexane to obtain a solution having a concentration of about 1mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Menthol RSin n-hexane to obtain a solution having a known concentration of about 1mg per mL.Transfer 5.0mLof this solution and 5.0mLof Internal standard solutionto a 50-mLvolumetric flask,dilute with ether to volume,and mix.Combine 2.0mLof this solution and 2.0mLof ether in a suitable container,and mix.This Standard preparationhas a known concentration of about 0.05mg per mL.

Assay preparation— Transfer an accurately weighed quantity of Ointment,equivalent to about 5mg of menthol,to a 50-mLvolumetric flask,add 5.0mLof Internal standard solution,dilute with n-hexane to volume,mix,and sonicate.Using a suitable syringe attached firmly to a 25-×12.5-mm chromatographic cartridge containing packing L4,force 2.0mLof the solution through the cartridge at a rate of 1mLper 12seconds.Wash the cartridge at the same rate with two 5-mLportions of n-hexane,and discard the washings.Force two 2.0-mLportions of ether through the cartridge,combine the ether eluates in a suitable container,and mix.

Chromatographic system (see Chromatography á621ñ)—The gas chromatograph is equipped with a flame-ionization detector and contains a 2-mm ×1.8-m column packed with 10%phase G16on support S1AB.The column temperature is maintained isothermally at about 170,the injection port temperature is maintained at about 260,and the detector block temperature is maintained at about 240.Dry helium is used as the carrier gas at a flow rate of about 50mLper minute.

System suitability— Chromatograph three injections of the Standard preparation,and record the peak responses as directed for Procedure:the retention time of menthol is about 0.7relative to decanol;the resolution,R,between the 2peaks is not less than 2.5;and the relative standard deviation of the ratio of the peak response obtained with menthol to that obtained with decanol is not more than 2%.

Procedure— Separately inject equal volumes (about 2µL)of the Assay preparationand the Standard preparationinto the gas chromatograph,and measure the peak responses for menthol and decanol in each chromatogram.Calculate the quantity,in mg,of C10H20Oin the portion of Ointment taken by the formula: in which Cis the concentration,in mg per mL,of USP Menthol RSin the Standard preparation;and RUand RSare the peak response ratios of menthol to decanol obtained from the Assay preparationand the Standard preparation,respectively.