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: Place a volume of Oral Solution, equivalent to about 100 mg of oxtriphylline, in a 60-mL separator containing 1 mL of glacial acetic acid and 40 mL of chloroform. Shake for 1 minute, allow the phases to separate, and filter the lower phase through dry cotton into a 100-mL beaker. Transfer a portion of the chloroform solution, equivalent to about 10 mg of oxtriphylline, to a porcelain dish, and evaporate on a steam bath with the aid of a current of dry air to dryness. Add 1 mL of hydrochloric acid and 100 mg of potassium chlorate, evaporate on a steam bath to dryness, and invert the dish over a vessel containing a few drops of 6 N ammonium hydroxide: the residue acquires a purple color.

Mobile phase, Standard preparation, System suitability preparation, and Chromatographic system— Prepare as directed in the Assay under Oxtriphylline Delayed-release Tablets.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 100 mg of oxtriphylline, to a 1000-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Oxtriphylline Delayed-release Tablets. Calculate the quantity, in mg, of C7H8N4O2 in each mL of the Oral Solution taken by the formula: in which V is the volume, in mL, of Oral Solution taken to prepare the Assay preparation, and the other terms are as defined therein.