In Situ Column Modification of Silica Monoliths With Pentafluorophenyl Ligands for the HPLC Separation of Eight Tocochromanols

Abstract

Silica-based monoliths offer higher separation efficiency per unit pressure drop compared to particle-packed columns. Their application is limited by the commercial availability of different column chemistries. Pentafluorophenyl ligands enable hydrogen bonding, dipole–dipole, π–π, and hydrophobic interactions, facilitating the separation of various compounds. This study employs an in situ modification procedure, namely, the silylation reaction to bond these specific ligands to silica-based monoliths. Three chromolith silica monoliths were modified with three different PFP-moieties. The silica monolith was successfully modified in situ with 3-(pentafluorophenyl)propylmethyldichlorosilane and the separation behavior was compared to a particle-packed pentafluorophenyl column and a C18 monolith for the separation of tocochromanols (Vitamin E-related compounds). The C18 monolith failed to resolve all the tocochromanols, while the PFP particle-packed column and monolith were comparable under their optimized conditions. The PFP monoliths conditions are as follows: isocratic water-methanol (22:78, v/v) mobile phase, with a flow rate of 1.45 mL/min, at 15°C. Further chromatographic comparisons between the PFP particle-packed column to the PFP monolith included selectivity studies under isocratic conditions via the linear solvent strength model, which also showed comparable selectivity behavior. Differences in selectivity was demonstrated for a gradient separation of eight compounds to represent a relatively more complex separation. In addition, backpressure and Van Deemter comparisons were performed, and it was demonstrated that the main PFP monolith advantages were obtained near the pressure maximum of the PFP particle-packed column at 3.5 mL/min, comparable efficiencies were obtained, and the PFP monolith decreased the backpressure by one-third.