An Investigation of the Role of Silica Substrate Topography on the Tailing and Broadening of Basic Compounds in HPLC

Abstract

Atomic force microscopy, Fourier transform infrared spectroscopy, high performance liquid chromatography, and UV-vis spectroscopy were used to study commercial and locally produced chromatographic silica substrates. Previous work has correlated the presence of isolated silanols with the peak tailing and broadening of basic analytes, and studies have shown enhanced adsorption of these analytes to isolated silanols at topographic features on planar, fused silica coverslips. This work provides evidence that similar topographical features exist on chromatographic substrates.This work is the first, to our knowledge, to provide nanoscale, topographic data on commercial silica. Three products; by Agilent, Merck, and Waters all have surface features that are similar in size and shape to the features on planar coverslips. The Agilent product also has seams where the individual colloids are fused together to form the bulk particle. These seams may not be fully silylated due to steric hindrance. Neglecting these features, the bulk of the silica surface is as smooth as fused silica.The AFM, FTIR, and HPLC data all indicate that the Waters silica had roughly twice the abundance of isolated silanols as the Merck product. The HPLC data for the Agilent material exhibited fronting and could not be modeled, but the FTIR data indicated that it had the same isolated silanol abundance as the Merck product.These same methods were used to characterize nonporous silica particles produced in our laboratory to be used as substrates for a variety of separation techniques. From the initial silica particles to the final colloidal crystal, AFM, FTIR, and UV-Vis data used to characterize these materials is presented. A method to determine the refractive index of the silica particles was developed and the changes in refractive index as the material is processed are also shown.Separation of three dye species and another separation of three peptide chains was done on a 2D colloidal crystal of these silica particles. The colloidal crystal outperformed a commercial, silica monolithic product both in terms of peak asymmetry and in resolving the analytes. This preliminary work shows the promise of this material as a substrate for chromatographic separations.