(Official January 1, 2007)

pH 3.6 Buffer— Dissolve 0.900 g of anhydrous dibasic sodium phosphate and 1.298 g of citric acid monohydrate in water to make 1000 mL.

pH 7.0 Buffer— Dissolve 5.68 g of anhydrous dibasic sodium phosphate and 3.63 g of monobasic potassium phosphate in water to make 1000 mL.

Mobile phase— Prepare a suitable mixture of pH 3.6 Buffer and acetonitrile (9:1). Pass through a membrane filter having a 10-µm or finer porosity, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Transfer 750 mg of salicylic acid to a 100-mL volumetric flask, dissolve in 10 mL of methanol, dilute with pH 7.0 Buffer to volume, and mix.

Standard preparation— Transfer about 25 mg of USP Cefazolin RS, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with pH 7.0 Buffer to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask, add 5.0 mL of Internal standard solution, dilute with pH 7.0 Buffer to volume, and mix.

Assay preparation— Proceed as directed for Standard preparation, except to use about 25 mg of Cefazolin, accurately weighed.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.0-mm × 30-cm column that contains 10-µm packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.7 for salicylic acid and 1.0 for cefazolin; the resolution, R, between the analyte and internal standard peaks is not less than 4.0; the column efficiency is not less than 1500 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 Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C14H14N8O4S3 in the portion of Cefazolin taken by the formula: in which C is the concentration, in mg per mL, of USP Cefazolin RS, calculated on the anhydrous basis, in the Standard preparation; and RU and RS are the peak response ratios of cefazolin to the internal standard obtained from the Assay preparation and the Standard preparation, respectively.