A method for the determination of minoxidil in hair-regrowth formulations by micellar electrokinetic capillary chromatography

A method based on micellar electrokinetic capillary chromatography (MEKC) was developed for determination of minoxidil in Rogaine and competing products. The original intent of the work was to offer an orthogonal means to HPLC for testing illicit imitations of Rogaine. However, because the patent has since expired, we offer the procedure as a confirmatory measure to HPLC for assay of generic minoxidil products. The MEKC procedure complements an earlier method based on free solution capillary electrophoresis (FSCE), designed to the same end. Validation was carried out on both a Dionex CES-1, which utilizes gravity injection, and a PE-ABI 270HT, which employs vacuum injection. The procedure was validated for both active pharmaceutical ingredient and for minoxidil solutions. The run buffer is pH 7.0, 20 mM sodium phosphate, 20 mM sodium dodecyl sulfate, with 10% isopropanol; the internal standard is dl-tryptophan. The method bears the attributes of simplicity, ease of use, and short analysis time (12 min). It is selective with respect to known process and degradation impurities. High efficiency was achieved on the CES-1, with a plate count exceeding 200,000 for minoxidil at an elution time of 9 min. Although slight differences in performance were noted across the two instruments, results on both were in conformance with modern day validation expectations. Comparison of MEKC with HPLC resulted in slightly higher values for the former, but all results met registration specifications and internal targets.