Columns packed with superficially porous particles (SPPs) have created considerable excitement over the last few years. Indeed, this column technology manifests the advantages of fully porous material (loading capacity, retention) and some beneficial properties of nonporous particles (kinetic performance). This review provides an updated overview of the theory behind the success of SPP technology, trends, benefits, and limitations. It also summarizes the latest developments of sub-2-µm SPPs and instrumental constraints associated with their use. Finally, it describes several applications to illustrate the performance and the universal applicability of these newly engineered particles.
Superficially porous particles (SPPs) (also called core–shell, fused-core shell, partially porous, pellicular, or solid-core) are made of a solid, nonporous silica core surrounded by a porous shell layer with similar properties to those of the fully porous materials used in conventional high performance liquid chromatography (HPLC) columns. The "fused-core" terminology was originally introduced by Jack Kirkland to describe the manufacturing procedure that "fuses" a porous silica layer onto a solid silica particle (1).
The very high efficiency achieved on columns packed with sub-3-µm SPPs, combined with convenient operating conditions (modest back pressures and the ability to use conventional HPLC instruments), has generated significant interest in the chromatographic community and widespread applications in many fields (2). Columns packed with sub-3-µm SPPs rival the efficiency of columns packed with sub-2-µm fully porous particles, but the former generate only half the back pressure. As a result, practitioners can use such columns on regular HPLC equipment, leading to the initial interest in these materials and their successful application. Moreover, further performance improvement is possible with very fine SPPs (1.3–1.7 µm), though the use of ultralow-dispersion ultrahigh-pressure liquid chromatography (UHPLC) systems (for example, extracolumn peak variance σec 2 < 3 µL2) is mandatory. Faster analysis and higher efficiency is always desirable in liquid chromatography (LC), particularly to pharmaceutical scientists or researchers in life science wishing to attain higher productivity in the laboratory or more accurate analysis (higher resolution) of very complex samples. Reducing analysis time while maintaining resolution requires high kinetic performance (more separation power per unit time) using smaller particles, better particle morphology, or both.The initial intent of applying SPPs was to enhance kinetic performance in the analysis of large biomolecules such as therapeutic proteins. The rationale behind this concept was to improve column efficiency by shortening the diffusion path that molecules have to travel and, thus, improve their mass transfer kinetics (3,4). Shell-type (pellicular) particles were first developed by Horváth and colleagues in the late 1960s for the analysis of large molecules in ion-exchange mode (5). Shortly afterward, Kirkland demonstrated that superficially porous particles (pellicular materials) with 30–40 µm diameters could provide much better separations than totally porous ones (6). In 2001, a new column for fast protein or peptide analysis was introduced that was packed with a 5-µm SPP with a shell thickness of 0.25 µm.