A:Infrared Absorption á197Kñ

B: Ignite about 100mg:a 1in 20solution of the residue in acetic acid responds to the tests for Sodium á191ñ.

Diluent— Dissolve 5.44g of monobasic potassium phosphate in water to make 2000mLof solution,and adjust with 8Npotassium hydroxide to a pHof 5.0±0.1.

Mobile phase— Prepare a suitable filtered mixture of Diluentand acetonitrile (1500:500).Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).Increasing the acetonitrile concentration decreases the retention time of dicloxacillin.

Standard preparation— Dissolve an accurately weighed quantity of USP Dicloxacillin Sodium RSin Diluentto obtain a solution having a known concentration of about 1.1mg per mL.[NOTE—Use this Standard preparation promptly,or refrigerate and use on the day prepared.]

Assay preparation— Transfer about 230mg of Dicloxacillin Sodium,accurately weighed,to a 200-mLvolumetric flask,add Diluentto volume,and mix.Stir with the aid of a magnetic stirrer for 5minutes to assure dissolution of the specimen.[NOTE—Use this Assay preparation promptly,or refrigerate and use on the day prepared.]

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 225-nm detector and a 4.6-mm ×25-cm column containing packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the responses as directed for Procedure:the capacity factor,k¢,for dicloxacillin is between 4and 11;the column efficiency is not less than 700theoretical plates;the tailing factor for the analyte peak is not more than 2.0;and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the area responses for the major peaks.Calculate the quantity,in µg,of dicloxacillin (C19H17Cl2N3O5S)in each mg of the Dicloxacillin Sodium taken by the formula: in which Cis the concentration,in mg per mL,of USP Dicloxacillin Sodium RSin the Standard preparation;Eis the dicloxacillin equivalent,in µg per mg,of USP Dicloxacillin Sodium RS;Wis the weight,in mg,of the portion of Dicloxacillin Sodium taken;and rUand rSare the dicloxacillin peak area responses obtained from the Assay preparation and the Standard preparation,respectively.