High-performance liquid chromatography of quinoidal imminium compounds derived from triphenylmethanes

Abstract

A series of eleven p -aminotriphenylmethane dyes have been studied by high-performance liquid chromatography (HPLC). The combined use of HPLC and spectrophotometry permits specific detection of these compounds in the visible range around 600 nm. As the high affinity of the imminium cations for the active sites of the hydrocarbonaceous stationary phase has presented difficulties for reversed-phase HPLC with pure solvents, organic electrolytes were added to the mobile phase to facilitate the elution of the components with improved selectivity, sensitivity (minimum detection limit, 0.1 μg/ml), and peak symmetry. The effects of chromatographic variables on the component retentivity were investigated. Retention times of the dye analytes decreased with increasing concentration of the added ionic reagent and with decreasing number of the hydrophobic alkyl substituents on the nitrogen atom. The influence of pH on the retention parameters appears to parallel that observed previously for cationic quaternary ammonium compounds. Among the acidic reagents employed, naphthalenesulfonic acid yielded the most satisfactory results. The use of binary electrolyte systems invariably improved the chromatographic behavior of the imminium solutes analyzed. Results obtained with two different octadecylsilica columns have been compared.