Chromatographic purity
Mobile phase
and
Chromatographic systemPrepare 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:
100S / (S + A),
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%.
Assay
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 C
9H
15N
5O in the portion of Minoxidil taken by the formula:
20C(RU / RS),
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.