Probing strong adsorption of solute onto cis-silica gel by fluorescence correlation imaging and single-molecule spectroscopy under RPLC conditions

Understanding molecular adsorption at a chromatographic interface is of great interest for addressing the tailing problem in chemical separations. Single-molecule spectroscopy and confocal fluorescence correlation imaging are used to study the adsorption sites of C-18 silica beads under RPLC chromatographic conditions. The experiments show that cationic molecule rhodamine 6G laterally diffuses through the chromatographic interface of a C18 hydrocarbon monolayer and acetonitrile with occasional reversible strong adsorptions. Fluorescence correlation imaging extracts the rare strong adsorption events from large data sets, revealing that the strong adsorption sites are randomly distributed throughout the silica beads. Virtually every imaging pixel of silica beads adsorbs molecules. Single-molecule spectroscopy of the 584 strong adsorption events observed indicates that the strong adsorptions persist on the time scales from several milliseconds to seconds, having an average desorption time of 61 ms. The strong adsorption events are rare, comprising 0.3% of the total observation time. The sizes of strong adsorption sites are within the optical resolution of confocal imaging.