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

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

Procedure— Inject about 10µLof Test solutioninto 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 Sis the sum of the areas of the minor component peaks detected,and Ais 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 3.0g of docusate sodium per liter of solution,and mix.Adjust with perchloric acid to a pHof 3.0,filter,and degas.

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

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

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

Chromatographic system (see Chromatography á621ñ)—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 1mLper 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 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times are about 0.8for the internal standard and 1.0for minoxidil.Calculate the quantity,in mg,of C9H15N5Oin the portion of Minoxidil taken by the formula: in which Cis the concentration,in mg per mL,of USP Minoxidil RSin the Standard preparation,and RUand RSare the peak response ratios obtained from the Assay preparationand the Standard preparation,respectively.