(Official January 1, 2007)

0.005 M Tetrabutylammonium hydroxide— Dilute 6.6 mL of a 40% solution of tetrabutylammonium hydroxide with water to obtain 1800 mL of solution. Adjust with 1 M phosphoric acid to a pH of 5.0 ± 0.1, dilute with water to 2000 mL, and mix.

Mobile phase— Prepare a filtered and degassed mixture of 0.005 M Tetrabutylammonium hydroxide and acetonitrile (1650:350). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Quantitatively dissolve accurately weighed quantities of USP Ampicillin RS and USP Sulbactam RS in Mobile phase to obtain a solution having known concentrations of about 0.6 mg of ampicillin per mL and 0.3 mg of sulbactam per mL. [NOTE—Inject this solution promptly.]

Resolution solution— Prepare a solution of USP Sulbactam RS in 0.01 N sodium hydroxide containing 0.3 mg per mL, and allow to stand for 30 minutes. Adjust with phosphoric acid to a pH of 5.0 ± 0.1. Transfer 5 mL of the solution to a 25-mL volumetric flask, add 4.25 mL of acetonitrile, dilute with 0.005 M Tetrabutylammonium hydroxide to volume, and mix. Transfer 1 mL of this solution to a second 25-mL volumetric flask, add 15 mg of USP Ampicillin RS, dilute with Mobile phase to volume, and mix. [NOTE—Inject this solution promptly.]

Assay preparation 1— Mix the contents of a container of Ampicillin and Sulbactam for Injection. Quantitatively dissolve an accurately weighed portion of the powder in Mobile phase to obtain a solution having a concentration of about 1 mg of the powder per mL. [NOTE—Inject this solution promptly.]

Assay preparation 2 (where it is represented as being in a single-dose container)— Constitute a container of Ampicillin and Sulbactam for Injection with a volume of water, accurately measured, corresponding to the volume of solvent specified in the labeling. Withdraw the total withdrawable contents from the container, using a suitable hypodermic needle and syringe, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution containing about 0.6 mg of ampicillin per mL and 0.3 mg of sulbactam per mL. [NOTE—Inject this solution promptly.]

Assay preparation 3 (where the label states the quantities of ampicillin and sulbactam in a given volume of constituted solution)— Constitute a container of Ampicillin and Sulbactam for Injection with a volume of water, accurately measured, corresponding to the volume of solvent specified in the labeling. Dilute an accurately measured volume of the constituted solution quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution containing about 0.6 mg of ampicillin per mL and 0.3 mg of sulbactam per mL. [NOTE—Inject this solution promptly.]

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 230-nm detector and a 4-mm × 30-cm column containing packing L1. The flow rate is about 2 mL per minute. Chromatograph the Resolution solution, and record the responses as directed for Procedure: the relative retention times are about 0.7 for ampicillin and 1.0 for sulbactam alkaline degradation product; and the resolution, R, between ampicillin and sulbactam alkaline degradation product is not less than 4.0. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the relative retention times are about 0.35 for ampicillin and 1.0 for sulbactam; the column efficiency determined from the sulbactam peak is not less than 3500 theoretical plates; the tailing factor is not more than 1.5; and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the appropriate Assay preparation into the chromatograph, record the chromatograms, and measure the areas for the major peaks. Calculate the quantities, in µg, of ampicillin (C16H19N3O4S) and of sulbactam (C8H11NO5S) in the portion of Ampicillin and Sulbactam for Injection taken by the same formula: in which CS is the concentration, in mg per mL, of the appropriate USP Reference Standard in the Standard preparation; P is the assigned content, in µg per mg, of the appropriate USP Reference Standard; CU is the concentration, in mg per mL, of Ampicillin and Sulbactam for Injection in Assay preparation 1, based on the weight, in mg, of powder removed from the container and the extent of dilution; and rU and rS are the peak areas for the appropriate analyte obtained from Assay preparation 1 and the Standard preparation, respectively. Calculate the quantities of ampicillin (C16H19N3O4S) and of sulbactam (C8H11NO5S) withdrawn from the container, or in the volume of constituted solution taken by the same formula: in which L is the labeled quantity, in mg, of ampicillin or sulbactam, as appropriate, in the container or in the volume of constituted solution taken; D is the concentration, in mg per mL, of ampicillin or sulbactam in Assay preparation 2 or Assay preparation 3, on the basis of the labeled quantity, in mg, of ampicillin or sulbactam, as appropriate, in the container and the extent of dilution; rU and rS are the peak areas for the appropriate analyte obtained from Assay preparation 2 or Assay preparation 3 and the Standard preparation, respectively; and the other terms are as defined above.