Critical differences in chromatographic properties of silica- and polymer-based monoliths

Chromatographic analytical columns containing porous monolithic beds based on cross-linked polymers and derivatized silica have now been commercially available for several years and, despite some apparent conceptual similarities, are marketed and utilized for quite different chromatographic applications. While this distinction is well-accepted by users, the fundamental differences in chromatographic behavior of these materials that lead to this clear distinction in their primary application areas have not yet been systematically studied. To this end, the present experimental study investigates differences in the apparent chromatographic characteristics when using small molecules with commercially available monolithic reversed-phase analytical columns based on poly(styrene-co-divinyl benzene) and C18-derivatized silica. Relevant practical information is obtained from measurements made by “arrested elution” of non-retained and retained solutes and chromatographic elution performance across a wide range of retention factors with a set of structurally similar small molecules. Observations of apparent diffusion probed with “arrested elution” experiments and mass transport inferred from the observed efficiency at increased flow velocity in the monolithic structures (both under retained and non-retained conditions) lead to the conclusion that fundamentally different solute transport behavior is operative. The silica-based monolithic materials are used to establish a “reference” for comparison to observations with cross-linked porous polymeric monolithic materials. Despite the differences in morphology, chromatographic properties have their origin in the underlying physical structure of pore space. The derivatized surfaces in silica-based materials have their counterpart in pore-fluid gel interfaces in polymeric monoliths. The pore-fluid gel interfaces have their origin in varying solvation of polymer by eluent components. Consequently, they allow varying permeation of small molecules into the solvated polymer via partition. The traversing of small molecules through the polymer monolith's complex nanoscale physical structure plays a key factor when rationalizing any chromatographic performance as seen in the slopes of plate height curves which vary dramatically with mobile phase composition and solute identity.