Annealing of silica to reduce the concentration of isolated silanols and peak tailing in reverse phase liquid chromatography.

Non-porous, colloidal silica particles were annealed at three different temperatures, 800, 900 and 1050 °C. The adsorption of lysozyme, a probe of surface roughness, was consistent with progressively reduced surface roughness as temperature increased. The heat treated silica particles were rehydroxylated and then used to pack UHPLC columns. The cationic protein lysozyme was used to probe silanol activity, which exhibited progressively less tailing as the annealing temperature increased. FTIR spectroscopy confirmed that the abundance of isolated silanols on the surface was reduced by annealing at 900 °C or 1050 °C. FTIR also revealed that there was markedly increased hydrogen bonding of the isolated silanols to neighbors after rehydroxylation. These results combine to support the hypothesis that (a) isolated silanols on silica cause tailing in RP-LC and (b) nonplanar topography gives rise to isolated silanols.

Sintered silica colloidal crystals with fully hydroxylated surfaces.

Silica colloidal crystals require multiple processing steps before they are useful materials in analytical applications, such as chemical separations, microarrays, sensors, and total internal reflection microscopy. These chemical processing steps include calcination, sintering, surface rehydroxylation, and chemical modification, but these steps have not been fully characterized for colloidal crystals. Silica particles of nominally 200 nm in diameter were prepared, and FTIR, SEM, UV-visible spectroscopy, and refractive index measurements were used to study the changes in chemical composition, particle size, and particle density throughout the process. The final material is shown to be a durable, crack-free crystal of solid particles bearing a fully hydroxylated surface of silanols, which can then be chemically modified.

Single-molecule probing of adsorption and diffusion on silica surfaces.

Single-molecule spectroscopy has emerged as a valuable tool in probing kinetics and dynamic equilibria in adsorption because advances in instrumentation and technology have enabled researchers to obtain high signal-to-noise ratios for common dyes at room temperature. Single-molecule spectroscopy was applied to the study of an important problem in chromatography: peak broadening and asymmetry in the chromatograms of pharmaceuticals, peptides, and proteins. Using DiI, a cationic dye that exhibits the same problematic chromatographic behavior, investigators showed that the adsorption sites that cause chromatographic problems are located at defects on the silica crystal surface.

Probing topography and tailing for commercial stationary phases using AFM, FT-IR, and HPLC.

Atomic force microscopy was used to study surface characteristics of three chromatographic silica products: Agilent Zorbax SB300, Waters Symmetry 300, and Merck Chromolith. Each is modified with a monomeric C18 monolayer. Both topographic and adhesive force measurements were made for each product. Topographical images revealed that all three materials are as smooth as glass on the scale of 100 nm and below. Adhesive forces for all three materials were much lower and much more uniform than for chemically modified fused silica. FT-IR spectra for all three materials showed a low abundance of isolated silanols, thus explaining the low adhesion. Chromatograms of a cationic dye, 1,1'-didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), ranging in concentration from 1 to 300 microM were obtained for each column. All three materials exhibited classic nonlinear tailing; the Zorbax exhibited fronting as well. Chromatographic simulations were performed for the Symmetry and Chromolith products to determine the number of strong adsorption sites. The AFM, FT-IR, and HPLC were all consistent in indicating that the Chromolith material had half as many strong adsorption sites as the Symmetry material. The Zorbax material exhibited a number of isolated silanols that was comparable to the other materials, yet its adhesive force suggested a less adsorptive material, and its chromatographic performance suggested a more adsorptive material. Its topography is discussed as a possible reason for its anomalous chromatographic behavior.

High-speed electroseparations inside silica colloidal crystals.

Crystals made from 200 nm silica colloids are hardened and chemically modified with chlorodimethyloctadecylsilane for use in electrically driven, reversed-phase separations. A van Deemter plot reveals extremely narrow peak widths for the separation of a cationic hydrophobic dye, DiI, with both the A and C terms 10-fold smaller than those for a conventional HPLC column. Electrically driven separations are demonstrated to be achieved in less than 10 s for three dyes differing in hydrophobicity and also for three peptides differing in electrophoretic mobility. The results show that these media are promising for high-speed separations.