A: The retention time exhibited by theophylline in the chromatogram of the Assay preparation corresponds to that of theophylline in the chromatogram of the Standard preparation, as obtained in the Assay.

B: Transfer a quantity of finely ground Tablets, equivalent to about 100 mg of oxtriphylline, to a suitable test tube, add 10 mL of methanol, shake on a vortex mixer for several minutes, and filter to obtain the test solution. Apply 10 µL of the test solution and 10 µL of a Standard solution of USP Oxtriphylline RS in methanol containing 10 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture. Allow the applications to dry, and develop the chromatogram in a solvent system consisting of a mixture of chloroform, alcohol, and formic acid (88:10:2) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Observe the plate under short-wavelength UV light: the principal spot obtained from the test solution corresponds in color, size, and RF value to that obtained from the Standard solution.

(Official January 1, 2007)

Mobile phase— Dissolve 6.8 g of monobasic potassium phosphate in water to make 1000 mL, and adjust with 0.1 N potassium hydroxide to a pH of 5.8 ± 0.1. Prepare a filtered and degassed mixture of this solution and methanol (4:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Oxtriphylline RS in water, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 0.1 mg per mL.

Assay preparation— Place 10 Tablets in a 1000-mL volumetric flask, and add about 700 mL of water. Heat on a steam bath, with occasional shaking, until the Tablets have disintegrated. Cool to room temperature, dilute with water to volume, mix, and filter. Transfer an accurately measured volume of this specimen solution, equivalent to about 20 mg of Oxtriphylline, to a 200-mL volumetric flask, dilute with water to volume, and mix.

System suitability preparation— Dissolve suitable quantities of USP Oxtriphylline RS and theobromine in water to obtain a solution containing about 0.6 mg and 0.3 mg per mL, respectively. Dilute this solution quantitatively, and stepwise if necessary, with water to obtain a solution containing about 60 µg of USP Oxtriphylline RS per mL and about 30 µg of theobromine per mL.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 275-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation and the System suitability preparation, and record the peak responses as directed for Procedure: the resolution, R, between the theobromine and theophylline peaks is not less than 3.0, and the relative standard deviation for replicate injections of the Standard preparation is not more than 2.0%. The relative retention times are about 0.7 for theobromine and 1.0 for theophylline.

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. [NOTE—The major peaks recorded in the chromatograms represent the theophylline moiety of oxtriphylline.] Calculate the quantity, in mg, of C7H8N4O2 per Tablet taken by the formula: in which 180.17 and 283.33 are the molecular weights of anhydrous theophylline and oxtriphylline, respectively, C is the concentration, in µg per mL, of USP Oxtriphylline RS in the Standard preparation, V is the volume, in mL, of specimen solution taken for the Assay preparation, and rU and rS are the theophylline peak responses obtained from the Assay preparation and the Standard preparation, respectively.