Stationary phases, multifunctional

Wei et al. studied the separation of amines (aniline, ephedrine, codeine, cocaine, thebaine) and quaternary ammonium compounds (berberine, jatrorrhi-zine) on a bare silica stationary phase [44], A thorough study of the separation mechanism revealed a complex multifunctional mechanism. Contributions from differential electrophoretic migration were superimposed on hydrophobic, cation-exchange, and normal-phase interactions. Retention was highly dependent on the pH (optimal pH 8.3), ionic strength, and the amount of organic modifier. As the content of acetonitrile exceeded 80%, retention was consistent with a normal-phase mechanism. 

Limitations for the applicability of SEC for certain molecules are the high shear forces due to the dense column packing, which may influence molecular assemblies and lead to degradation. Furthermore, the column packing material can induce unwanted interactions between the stationary phase and the molecules, thus shifting the separation mechanism from pure size exclusion towards separation by enthalpic interactions. Especially in cases of multifunctional polymers like dendritic molecules with a high number of end groups, delayed elution or complete adsorption can be observed. 

As a consequence of the multifunctional nature of the sorption parameter recent developments are directed to testing schemes using HPLC with various stationary phases. The basic principle is to determine separately the different interactions involved in sorption on standardized material, and then to recombine the data from the individual measurements for a comprehensive sorption parameterization. Until the experimental sorption data will be determined by a stringently standardized method, and the values refer to a consistently defined endpoint, sorption modelling must remain a formalization of empirical relationships. Accordingly any QSAR approach to soil sorption is principally restricted by the problem of determining precisely which activity is accounted for ... 

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