Band profiles of reacting acido-basic compounds with water-methanol eluents at different pHsWS and ionic strengths in reversed-phase liquid chromatography

Overloaded band profiles of aniline (25 μL injection of a 30.5 mM solution) eluted with a methanol-aqueous buffer solution (30/70, v/v) were recorded at the exit of a 150mm×4.6 mm column packed with 3.5 μm XBridge-C18porous particles. The pHWS of the mobile phase was adjusted with phosphate (pHWS∼2.7 and pHWS∼7.5) or acetate buffers (pHWS∼5.3) of different concentrations (11, 56, and 278 mM). The elution times and profiles of the bands observed at low ionic strength were succesfully accounted for using the extended Debye-Hückel theory to estimate the activity coefficients of the ions in the bulk phase and a simple non-competitive Langmuir adsorption model, the adsorption of pure aniline or pure anilinium onto XBridge-C18 being described by a Langmuir isotherm. The band profiles were calculated using this adsorption model and the equilibrium-dispersive model of chromatography. The calculated and the experimental band profiles are in excellent agreement at all buffer pH and ionic strength. This demonstrates that the elution times and the band profiles are controlled by the chromatographic dilution process and by the reaction of aniline with the buffer solution.