25 mM Monobasic sodium phosphate buffer— Prepare a 25 mM monobasic sodium phosphate solution, and adjust with phosphoric acid to a pH of 2.5.

Mobile phase— Prepare a solution of 25 mM Monobasic sodium phosphate buffer and acetonitrile (97.5:2.5). Filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Sample diluent— Prepare a solution of water, acetonitrile, and methanol (80:15:5).

Internal standard solution— Prepare a 4 mg per mL aqueous solution of hypoxanthine.

Standard stock preparation— Dissolve an accurately weighed quantity of USP Ganciclovir RS in water to obtain a concentration of about 1.0 mg per mL.

Standard preparation— Transfer an appropriate volume of Standard stock preparation to a suitable container, and add an appropriate volume of Internal standard solution to obtain a solution having known concentrations of 6 µg per mL and 4 µg per mL, respectively.

Assay preparation— Transfer about 1 mL of Oral Suspension from each bottle to a plastic weighing cup, and weigh to determine density. [note—The exact volume of Oral Suspension taken from each bottle is calculated by the suspension density.] Transfer the suspension to a 100-mL volumetric flask, and add about 50 mL of water. Place the volumetric flask on a mechanical shaker for 30 minutes, and then dilute with water to volume. Transfer 0.6 mL from this solution and 1 mL of the Internal standard solution to a 100-mL volumetric flask, and dilute with water to volume.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 10-cm analytical column that contains 5-µm packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are 1.0 for ganciclovir and 0.75 for hypoxanthine; and the relative standard deviation for replicate injections is not more than 1.5%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of ganciclovir (C9H13N5O4) in the volume of Oral Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Ganciclovir RS in the Standard preparation; V is the volume, in mL, of Oral Suspension taken; and RU and RS are the peak response ratios of the analyte peak to the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.