Phosphate buffer solution— Accurately weigh 68 g of monobasic potassium phosphate, and transfer to a 500-mL volumetric flask. Dissolve in and dilute with water to volume.

Mobile phase— Prepare a suitable mixture of water, acetonitrile, Phosphate buffer solution, and glacial acetic acid (3600:360:40:4). Pass through a 0.45-µm nylon filter, and degas.

Standard preparation— Dissolve, with sonication, an accurately weighed quantity of USP Ampicillin RS in water to prepare a solution having 0.5 mg per mL. Pass through a 0.45-µm PTFE filter, discarding the first 3 mL of the filtrate.

Caffeine solution— Transfer about 30 mg of caffeine, accurately weighed, to a 50-mL volumetric flask. Add 25 mL of water, sonicate to dissolve, and dilute with water to volume. Pass through a 0.45-µm PTFE filter, discarding the first 3 mL of the filtrate.

System suitability solution— Prepare a solution of 1.0 mL of Caffeine solution and 9.0 mL of Standard preparation, and mix.

Assay preparation— Quantitatively dilute an accurately measured volume of Ampicillin for Injectable Suspension, constituted as directed in the labeling, with water to obtain a solution containing about 0.5 mg per mL. Pass through a 0.45-µm PTFE filter, discarding the first 3 mL of the filtrate.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm analytical column that contains 10-µm packing L1. The flow rate is about 2.0 mL per minute. The column temperature is maintained at 40. Chromatograph the System suitability solution and the Standard preparation, and record the peak responses as directed for Procedure: the order of elution is ampicillin followed by caffeine; the resolution, R, between ampicillin and caffeine is greater than 2; the column efficiency is not less than 2000 theoretical plates for the ampicillin peak; the tailing factor is not greater than 1.4; and the relative standard deviation for replicate injections of the Standard preparation 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 areas of the major peaks. Calculate the quantity, in mg, of ampicillin (C16H19N3O4S) in each mL of the constituted solution of Ampicillin for Injectable Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Ampicillin RS in the Standard preparation; D is the dilution factor used in preparing the Assay preparation; and rU and rS are the average peak responses of the ampicillin peaks obtained from the Assay preparation and the Standard preparation, respectively.