Stationary phases with chemically bonded fluorene ligands: A new approach for environmental analysis of pi-electron containing solutes

For the separation of pi-electron-containing solutes by high-performance liquid chromatography (HPLC), a new stationary phase with a fluorene ligand chemically bonded to a silica gel surface was developed following a two-stage modification of silica gels with different pore sizes. This new phase was characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy and HPLC. Chromatographic properties were determined by the separation of three test mixtures, two containing polycyclic aromatic hydrocarbons (PAHs) and the third containing nitro explosives. The elution order of the PAHs is the same as in case of usual reversed-phases, while the elution of the nitro explosives is inverted because of strong pi-pi interactions between the electron-poor solutes and the electron-rich stationary phase. Solid-state NMR relaxation time measurements were used to study the dynamic behaviour of the bonded fluorene ligands. The results of solid-state NMR investigations are discussed in the context of various HPLC separations. Both chromatographic and spectroscopic properties show a dependence on the pore size of the starting silica gel matrix.