(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture containing 4 volumes of ammonium acetate solution (1 in 100) and 6 volumes of a mixture of methanol, acetonitrile, and glacial acetic acid (400:200:0.6). Adjust this solution to a pH of 6.6 ± 0.1 by the dropwise addition of glacial acetic acid, and make adjustments if necessary (see System Suitability under Chromatography 621), to obtain a retention time of about 5 minutes for the fentanyl peak.

Standard preparation— Dissolve an accurately weighed quantity of USP Fentanyl Citrate RS in water, and quantitatively dilute with water to obtain a solution having a known concentration of about 80 µg per mL.

Assay preparation— If necessary, dilute the Injection with water so that each mL contains the equivalent of about 50 µg of fentanyl.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 230-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak response as directed for Procedure: the tailing factor for the fentanyl 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 25 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 fentanyl (C22H28N3O) in each mL of the Injection taken by the formula: in which 336.48 and 528.59 are the molecular weights of fentanyl and fentanyl citrate, respectively; C is the concentration, in µg per mL, of USP Fentanyl Citrate RS in the Standard preparation; D is the dilution factor used to obtain the Assay preparation; and rU and rS are the peak responses for the fentanyl peak obtained from the Assay preparation and the Standard preparation, respectively.