A: Infrared Absorption 197M—

B: The retention time of the major peak for minoxidil in the chromatogram of the Assay preparation corresponds to that of the Standard preparation, as obtained in the Assay.

(Official January 1, 2007)

Mobile phase, Internal standard solution, Standard preparation , and Chromatographic system—Proceed as directed in the Assay under Minoxidil.

Assay preparation— Transfer an accurately measured volume of Topical Solution, equivalent to about 100 mg of minoxidil, to a 10-mL volumetric flask, dilute with Mobile phase to volume, and mix. Transfer 0.5 mL of this solution to a suitable vial, add 20.0 mL of Internal standard solution, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Minoxidil. Calculate the quantity, in mg, of minoxidil (C9H15N5O) in each mL of the Topical Solution taken by the formula: in which C is the concentration, in mg per mL, of USP Minoxidil RS in the Standard preparation, V is the volume, in mL, of the Topical Solution taken for the Assay preparation, and RU and RS are the ratios of the minoxidil peak to that of the internal standard obtained from the Assay preparation and the Standard preparation, respectively.