MOLECULAR-INTERACTIONS AT OCTADECYLATED CHROMATOGRAPHIC SURFACES

Interactions between fluorescent solutes and an octadecylated silica s urface are investigated using fluorescence emission spectra and quench ing techniques under conditions similar to those found in high-perform ance liquid chromatography. Pyrene, benzo[a]pyrene, fluorene, biphenyl , propyldansylamide, and decyldansylamide are used as the fluorescent probes, and potassium iodide and N,N-dimethylaniline are used as quenc hers. N,N-Dimethylaniline is a moderately retained quencher thought to probe deeper into the octadecylated surface than the ionic iodide sal t. Solvent-dependent fluorescence emission maxima, solvent-dependent f luorescence vibronic band intensities, and quencher access to the prob es are investigated using aqueous methanol mobile phase compositions r anging from 60 to 100% methanol. The resulting information is used to interpret differences in interfacial probe environments and to determi ne the location of the probes within the bonded phase layer. The data indicate that biphenyl and pyrene may remain in very nonpolar interfac ial probe environments deep hn the bonded phase layer over the mobile phase composition range tested. The fluorophore portion of both propyl dansylamide and decyldansylamide may reside in an interfacial environm ent, which becomes more polar as the water content in the mobile phase is increased. Benzo[a]pyrene apparently becomes exposed to the mobile phase as the water content in the mobile phase increases. This is tho ught to be due to the relatively large size of the solute molecule and the collapse of the octadecyl chains with increased solvent polarity. Fluorene appears to interact strongly with silanol groups. The result s are interpreted in light of the surface convolution and chain cluste r octadecylated chromatography surface models and are found to be more constant with the chain cluster model. Implications of the results to reversed phase high-performance liquid chromatography separations are also discussed.