A: The chromatogram of the Assay preparation obtained as directed in the Assay exhibits a major peak for oxacillin, the retention time of which corresponds to that exhibited in the chromatogram of the Standard preparation obtained as directed in the Assay.

B: It responds to the tests for Sodium 191.

Mobile phase— Dissolve 1.9 g of monobasic potassium phosphate in 700 mL of water. Add 300 mL of acetonitrile and 100 mL of methanol, and mix. Filter this solution through a 0.5-µm or finer porosity filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Oxacillin Sodium RS in water to obtain a solution having a known concentration of about 0.11 mg per mL. [note—Use this Standard preparation on the day prepared.]

Assay preparation— Transfer about 115 mg of Oxacillin Sodium, accurately weighed, to a 200-mL volumetric flask, add water to volume, and mix. Stir with the aid of a magnetic stirrer for 5 minutes to ensure dissolution of the specimen. Transfer 10.0 mL of this solution to a 50-mL volumetric flask, dilute with water to volume, and mix. [note—Use this Assay preparation on the day prepared.]

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 225-nm detector and a 4-mm × 30-cm column that contains packing L11. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the responses as directed for Procedure: the column efficiency is not less than 2000 theoretical plates, the tailing factor is not more than 1.6, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— [note—Use peak areas where peak responses are indicated.] 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 µg, of oxacillin (C19H19N3O5S) in each mg of the Oxacillin Sodium taken by the formula: in which C is the concentration, in mg per mL, of USP Oxacillin Sodium RS in the Standard preparation; E is the oxacillin equivalent, in µg per mg, of USP Oxacillin Sodium RS; W is the weight, in mg, of the portion of Oxacillin Sodium taken; and rU and rS are the oxacillin peak responses obtained from the Assay preparation and the Standard preparation, respectively.