Evaluation of HPLC columns: A study on surface homogeneity of chemically bonded stationary phases

The aim of the current work is to study the heterogeneity of the adsorbent surface on the basis of physicochemical investigations and chromatographic tests. A series of packing materials with octadecyl chains chemically bonded to a silica matrix were prepared for this purpose, The surface and structural properties of bare silica and silica-based octadecyl phases were characterized by porosimetry, elemental analysis, Si-29 CP/MAS NMR, etc. The most advanced characterization methods based on adsorption microcalorimetry (heat of wetting) measurements were employed to obtain information about the heterogeneity and topography of unmodified and modified silica gel. For the chromatographic study, these phases were evaluated on the basis of the retention data under non-aqueous conditions. A test series of solutes with various chemical properties, such as pK(a) values, was used. It was found that heterogeneity of the packing surface results in low HPLC resolution. Use of a non-aqueous mobile phase (n-hexane) reduces analytical interference by eliminating hydrophobic interactions between alkyl ligands and the analyte.