The Fractional Charge Approach in Ion‐Interaction Chromatography of Zwitterions: Influence of the Stationary Phase Concentration of the Ion Interaction Reagent and pH

Abstract

The fractional charge approach to Ion‐interaction chromatography (IIC) of zwitterions was used to quantitatively explain their retention behavior as a function of the Ion Interaction Reagent (H) concentration in the stationary phase and also to account for the influence of the zwitterion ionization degree, according to the mobile phase pH.

The theory was validated using two Hs and two reversed phase columns. The good predictive abilities of the retention equations lend strong support to the hypotheses made by the fractional charge approach in the IIC of zwitterions. For the investigated homologous series the increase in retention upon H addition can be quantitatively related to the estimated fractional charge. The estimates of the operative charges and bonded phase coverages are very reasonable since they make sense physically. This way, they offer a cross validation of the fractional charge approach, since they compare well with those obtained from the fitting of parallel retention data, obtained as a function of the mobile phase concentration of H. This confirms that retention modelling may instruct the chromatographer through the optimization procedure in the parameter space.

Keywords:

• Reversed‐phase ion‐interaction chromatography
• Zwitterion
• Retention model
• Ion interaction reagent surface concentration

Acknowledgments

Prof S. Fazzini and the teaching staff of ITIS Montani are gratefully acknowledged for the availability of the laboratories of the Chemistry Department of ITIS Montani.