Investigation of an alkylamine modified and pH-controlled mobile phase for separation of pyrazolone derivatives by high-performance liquid chromatography.

The separation of pyrazolone derivatives with different acid-base properties and polarities has been investigated in view of recent concepts about the retention mechanisms in reversed-phase liquid chromatography. It is shown that both the protonated (BH+) and uncharged (B) bases undergo a dual mechanism of retention due to solvophobic and silanophilic interactions. The latter are easily suppressed by the addition of an alkylamine modifier (dibutylamine) to the mobile phase. An ion-pairing mechanism is found to be valid in the retention of negatively charged (A-) pyrazolone derivatives, since the dibutylamine modifier (BH+) serves as an appropriate counter ion. The behaviour of these solutes can be predicted from the pH dependence of their capacity factors, and a new simple method is proposed for calculating the theoretical sigmoidal curves. It is shown that the elution order of the compounds can be changed by control of both the pH value and the modifier concentration in the mobile phase.