A: The IR absorption spectrum of a potassium bromide dispersion of it exhibits maxima only at the same wavelengths as that of a similar preparation of USP Phenobarbital RS. If a difference appears, dissolve portions of both the test specimen and the USP Reference Standard in a suitable solvent, evaporate the solutions to dryness, and repeat the test on the residues.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that of the Standard preparation, both relative to the internal standard, as obtained in the Assay.

pH 4.5 Buffer solution— Dissolve about 6.6 g of sodium acetate trihydrate and 3.0 mL of glacial acetic acid in 1000 mL of water, and adjust, if necessary, with glacial acetic acid to a pH of 4.5 ± 0.1.

Mobile phase— Prepare a filtered and degassed mixture of pH 4.5 Buffer solution and methanol (3:2), making adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Dissolve a sufficient quantity of caffeine in a mixture of methanol and pH 4.5 Buffer solution (1:1) to obtain a solution having a concentration of about 125 µg per mL.

Standard preparation— Dissolve about 20 mg of USP Phenobarbital RS, accurately weighed, in 15.0 mL of Internal standard solution. Sonicate if necessary.

Assay preparation— Transfer about 20 mg of Phenobarbital, accurately weighed, to a conical flask, add 15.0 mL of Internal standard solution, mix, and sonicate for 15 minutes. Filter through a membrane filter (0.5 µm or finer porosity) before use.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4-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 responses as directed for Procedure: the resolution, R, between the analyte and the internal standard peaks is not less than 1.2, the tailing factor for the analyte and the internal standard peaks is not greater than 2.0, 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. The relative retention times are about 0.6 for caffeine and 1.0 for phenobarbital. Calculate the quantity, in mg, of C12H12N2O3 in the portion of Phenobarbital taken by the formula: in which W is the weight, in mg, of USP Phenobarbital RS taken for the Standard preparation, and RU and RS are the peak response ratios obtained from the Assay preparation and the Standard preparation, respectively.