Mobile phase and Chromatographic system— Proceed as directed in the Assay.

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

Test solution— Use the Assay preparation.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the Test solution into 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 C is the concentration, in mg per mL, of guanine in the Standard solution; D is the concentration, in mg per mL, of acyclovir in the Test solution; and rU and rS are the guanine peak responses obtained from the Test solution and the Standard solution, respectively: not more than 2.0% is found.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed solution of 0.02 M acetic acid. Make adjustments if necessary (see System Suitability under Chromatography 621).

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

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

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

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

Chromatographic system (see Chromatography 621)— 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 3 mL per minute. Chromatograph System suitability preparation 1, and record the peak responses as directed for Procedure: the relative retention times are about 0.6 for guanine and 1.0 for acyclovir; the resolution, R, between guanine and acyclovir is not less than 2.0; and the relative standard deviation for replicate injections for acyclovir is 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%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into 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 C is the concentration, in mg per mL, of USP Acyclovir RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.