Mobile phaseand Chromatographic system— Proceed as directed in the Assay.

Standard solution— Dissolve an accurately weighed quantity of guanine in 0.1Msodium hydroxide,and dilute quantitatively,and stepwise if necessary,with 0.1Msodium hydroxide to obtain a solution having a known concentration of about 2.0µg per mL.

Test solution— Use the Assay preparation.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms,and measure the peak responses.Calculate the percentage of guanine in the portion of Ointment taken by the formula: in which Cis the concentration,in mg per mL,of guanine in the Standard solution;Dis the concentration,in mg per mL,of acyclovir in the Test solution;and rUand rSare the guanine peak responses obtained from the Test solutionand the Standard solution,respectively:not more than 2.0%is found.

Change to read:

Mobile phase— Prepare a filtered and degassed solution of 0.02Macetic acid.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve accurately weighed quantities of USP Acyclovir RSUSP28in 0.1Nsodium hydroxide,and dilute quantitatively,and stepwise if necessary,with 0.1Nsodium hydroxide to obtain a solution having known concentrations of about 0.1mg per mL.USP28

System suitability preparation 1— Dissolve accurately weighed quantities of USP Acyclovir RSand guanine in 0.1Nsodium hydroxide,and dilute quantitatively,and stepwise if necessary,with 0.1Nsodium hydroxide to obtain a solution having known concentration of about 0.1mg of each per mL.

System suitability preparation 2— Dissolve an accurately weighed quantity of guanine in 0.1Nsodium hydroxide,and dilute quantitatively,and stepwise if necessary,with 0.1Nsodium hydroxide to obtain a solution having a known concentration of about 2.0µg per mL.USP28

Assay preparation— Transfer an accurately weighed quantity of Ointment,equivalent to about 10mg of acyclovir,to a 100-mLvolumetric flask,dissolve in and dilute with 0.1Nsodium hydroxide to volume,and mix.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm ×25-cm column that contains packing L1.The flow rate is about 3mLper minute.Chromatograph System suitability preparation 1,USP28and record the peak responses as directed for Procedure:the relative retention times are about 0.6for guanine and 1.0for acyclovir;the resolution,R,between guanine and acyclovir is not less than 2.0;and the relative standard deviation for replicate injections for acyclovirUSP28is not more than 2.0%.Chromatograph System suitability preparation 2,and record the peak responses as directed for Procedure:the relative standard deviation for replicate injections is not more than 2.0%.USP28

Procedure— Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the peak responses.Calculate the quantity,in mg,of acyclovir (C8H11N5O3)in the portion of Ointment taken by the formula: in which Cis the concentration,in mg per mL,of USP Acyclovir RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.