Mobile phase and Chromatographic system—Prepare as directed in the Assay.

Test solution— Prepare a solution of Minoxidil in Mobile phase having a concentration of about 0.25 mg per mL.

Procedure— Inject about 10 µL of Test solution into the chromatograph, record the chromatogram, and measure the peak response for each component. Calculate the total percentage of impurities taken by the formula: in which S is the sum of the areas of the minor component peaks detected, and A is the area of the major component. The total of any impurities detected is not more than 1.5%.

Mobile phase— Prepare a solution consisting of a mixture of methanol, water, and glacial acetic acid (700:300:10), add 3.0 g of docusate sodium per L of solution, and mix. Adjust with perchloric acid to a pH of 3.0, filter, and degas.

Internal standard solution— Prepare a solution of medroxyprogesterone acetate in Mobile phase having a concentration of about 0.2 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Minoxidil RS in Internal standard solution to obtain a solution having a known concentration of about 0.25 mg per mL.

Assay preparation— Transfer about 5 mg of Minoxidil, accurately weighed, to a container, add 20.0 mL of Internal standard solution, and mix.

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 1 mL per minute. Chromatograph not less than four replicate injections of the Standard preparation, and record the peak responses as directed under Procedure: the relative standard deviation is not more than 2.0%, and the resolution, R, between the internal standard and minoxidil is not less 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 0.8 for the internal standard and 1.0 for minoxidil. Calculate the quantity, in mg, of C9H15N5O in the portion of Minoxidil taken by the formula: in which C is the concentration, in mg per mL, of USP Minoxidil RS in the Standard preparation, and RU and RS are the peak response ratios obtained from the Assay preparation and the Standard preparation, respectively.