A: To an amount of the Oral Solution equivalent to about 100 mg of dyphylline, add water to make 20 mL, mix, and add 2.0 mL of 2 N sodium hydroxide and 2 drops of potassium permanganate TS: a green color is produced.

B: Transfer a volume of Oral Solution, equivalent to about 100 mg of guaifenesin, to a 60-mL separator, add 10 mL of chloroform, shake for 30 seconds, and allow the layers to separate. Decant the lower (chloroform) layer through chloroform-washed cotton into a small beaker. Evaporate 1 mL of the extract, on a watch glass, on a steam bath to dryness. To the residue add 1 drop of formaldehyde TS and a few drops of sulfuric acid: a deep cherry-red to purple color is produced.

C: The retention times of the major peaks in the chromatogram of the Assay preparation correspond to those in the chromatogram of the Standard preparation, as obtained in the Assay.

Standard preparation, Test preparation, Chromatographic system, and Procedure— Proceed as directed in Alcohol content under Dyphylline Oral Solution: the alcohol content is between 90.0% and 110.0% of the labeled amount of C2H5OH.

(Official January 1, 2007)

Mobile phase— Prepare a suitable filtered and degassed mixture of 0.01 M monobasic potassium phosphate and methanol (79:21). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve accurately weighed quantities of USP Dyphylline RS and USP Guaifenesin RS in Mobile phase, and dilute quantitatively, and stepwise if necessary, with Mobile phase to obtain a solution having known concentrations of about 0.1 mg of guaifenesin and about 0.1J mg of dyphylline per mL, J being the ratio of the labeled amount of dyphylline to that of guaifenesin.

Resolution solution— Prepare a solution in Mobile phase containing in each mL about 0.1 mg each of dyphylline and guaifenesin and about 0.01 mg of guaiacol.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 100 mg of guaifenesin, to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix. Transfer 5.0 mL of this solution to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 230-nm detector, a guard column that contains packing L1, and a 4.6-mm × 15-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Resolution solution, and record the peak responses as directed for Procedure: the resolution, R, between the guaiacol and guaifenesin peaks is not less than 1.8, and the resolution, R, between the guaiacol and dyphylline peaks is not less than 9.0. The relative retention times are about 0.25 for dyphylline, 0.7 for guaiacol, and 1.0 for guaifenesin. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0% for both dyphylline and guaifenesin.

Procedure— Separately inject equal volumes (about 20 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 quantities, in mg, of dyphylline (C10H14N4O4) and guaifenesin (C10H14O4) in each mL of the Oral Solution taken by the formula: in which C is the concentration, in mg per mL, of the appropriate USP Reference Standard in the Standard preparation, V is the volume, in mL, of Oral Solution taken; and rU and rS are the peak responses of the corresponding analyte obtained from the Assay preparation and the Standard preparation, respectively.