Characterization of calixarene-bonded stationary phases

Calixarene-bonded stationary phases received growing interest in HPLC as stationary phases with special retention characteristics and selectivity. The commercially available unsubstituted and p-tert-butyl-substituted Caltrex(®) columns have been intensively studied and characterized in our workgroup. They can be used as reversed phases, yet they support additional interactions. Especially, their steric, polar and ionic properties differ from conventional alkyl-bonded phases. However, also the hydrophobic interaction shows differences since adsorption and partition interactions on or in a bonded layer of calixarenes are not similar to those of alkyl-bonded layers. The relative strength of the hydrophobic properties of the stationary phases has been found depending on the methanol concentration of the mobile phase. Generally, the dependencies of their interaction strengths on mobile-phase conditions, e.g. the change of the intensity of the hydrogen-bonding abilities with decreasing methanol content, are not similar from phase to phase either. This probably gives calixarene-bonded stationary phases enhanced suitability for analyses at extreme compositions of the mobile phase. An overview about the synthesis, retention and selectivity properties of Caltrex(®) columns is given here.