Investigation of the retention behaviour of steroids with calixarene-based stationary phases by modern NMR spectroscopy

A separation of four structurally similar steroids was performed on two calix[8]arene based stationary phases. It was observed that two of the steroids had a reversed retention order when the columns were changed. This implies that two different retention mechanisms are taking place when the analytes are separated. In order to evaluate the mechanism responsible for the reversal of the retention order an extensive study of the interactions between the stationary phases and the analytes was undertaken. By performing a series of transferred NOESY measurements in the suspended state valuable information was obtained about the strengths of the interactions between the analytes and the stationary phases. The magnitudes of the integrals from the trNOESY measurements showed a good correlation with the W values obtained from the chromatographic evaluation. The retention mechanism is based mainly on hydrophobic interactions, However, the reversal of the elution order was mainly due to an additional pi-pi interaction between one of the analytes and the stationary phase.