A: Infrared Absorption 197F.

B: Ultraviolet Absorption 197U—

Solution: 20 µg per mL.

Medium: methanol.

Standard preparation— Prepare a solution in chloroform having known concentrations of about 0.1 mg of USP Clofibrate RS and 0.03 mg of p-chlorophenol per mL. To 10.0 mL of this solution add 5.0 µL of tributyrin, and mix.

Test preparation— To 10.0 mL of Clofibrate add 5.0 µL of tributyrin, and mix.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a split injector with a 20:1 split ratio, a flame-ionization detector, and a 0.53-mm × 15-m column to the internal walls of which is bonded a 1.5-µm film of phase G1. The chromatograph is programmed to maintain the column temperature at 120 for 1 minute, then to increase the temperature at a rate of 5 per minute for 12 minutes, and finally to maintain a temperature of 180 for 9 minutes. Maintain the injection port at 210 and the detector block at 220. Helium is used as the carrier gas flowing at a rate of about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed under Procedure: the relative retention times are about 0.2 for p-chlorophenol, 0.55 for clofibrate, and 1.0 for tributyrin.

Procedure— [note—Use peak areas where peak responses are indicated.] Separately inject equal volumes (about 1 µL) of the Standard preparation and the Test preparation into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the percentage of each impurity, other than p-chlorophenol, in the Clofibrate taken by the formula: in which C is the concentration, in mg per mL, of USP Clofibrate RS in the Standard preparation, RU is the ratio of the response of each individual impurity peak (other than the p-chlorophenol peak) to that of the tributyrin peak obtained from the Test preparation, and RS is the ratio of the response of the clofibrate peak to that of the tributyrin peak obtained from the Standard preparation: the percentage of any impurity, other than p-chlorophenol, does not exceed 0.01%, and the total percentage of all such impurities does not exceed 0.12%.

0.1C(RU / RS)

Ion-exchange resin— To a beaker containing 75 mL of 1 N sodium hydroxide add about 3 g of a 50- to 100-mesh strongly basic styrene-divinylbenzene anion-exchange resin, and allow the mixture to stand for about 15 minutes, with occasional stirring. Wash the resin with water until the last washing is neutral to litmus paper, then wash with three 50-mL portions of methanol.

Ion-exchange column— Place a plug of glass wool in the base of a 1- × 15-cm ion-exchange tube, and transfer to the tube a sufficient amount of Ion-exchange resin, slurried in methanol, to produce a column bed height of from 6 cm to 8 cm.

Standard preparation— Dissolve an accurately weighed quantity of USP Clofibrate RS in methanol, and dilute quantitatively and stepwise with methanol to obtain a solution having a known concentration of about 20 µg per mL.

Assay preparation— Transfer about 200 mg of Clofibrate, accurately weighed, to a 100-mL volumetric flask, add methanol to volume, and mix. Transfer 10.0 mL of this solution to the Ion-exchange column, and collect the eluate in a 100-mL volumetric flask. Rinse the column with 25 mL of methanol, collect the rinsing in the volumetric flask, dilute with methanol to volume, and mix. Transfer 5.0 mL of this solution to a 50-mL volumetric flask, dilute with methanol to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparation and the Assay preparation in 1-cm cells at the wavelength of maximum absorbance at about 226 nm, with a suitable spectrophotometer, using methanol as the blank. Calculate the quantity, in mg, of C12H15ClO3 in the portion of Clofibrate taken by the formula: in which C is the concentration, in µg per mL, of USP Clofibrate RS in the Standard preparation; and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.