SEPARATION OF MINOXIDIL AND ITS INTERMEDIATES BY OVERPRESSURED LAYER CHROMATOGRAPHY USING A STATIONARY PHASE BONDED WITH TRICAPRYLMETHYLAMMONIUM CHLORIDE

The retention behaviour of minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxide) and its intermediates (2,4-diamino-6-hydroxypirimidine, 2,4-diamino-6-chloropyrimidine and 2,4-diamino-3-N-oxo-6-chloropyrimidine) was studied by using silica gel layers impregnated with tricaprylmethylammonium chloride (TCMA). The retention of the compounds increases with increasing concentration of TCMA adsorbed on the silica gel. The pH and the ionic strength of the eluents do not affect the retention at all. The retention of the solutes decreases with increasing methanol content of the eluent, because of the TCMA-desorbing effect of methanol. On the basis of these and earlier findings, it was concluded that no ion-pairing occurs during the separation. A monolayer is formed on the silica surface at 0.1-0.2 M TCMA concentration in the impregnating solution, and hydrophobic interactions play an important role in the separation mechanism.