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-Ám 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-Ám 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-Ám SPPs rival the efficiency of columns
packed with sub-2-Ám 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 Ám), though the use of ultralow-dispersion
ultrahigh-pressure liquid chromatography (UHPLC) systems (for example, extracolumn peak variance σec
2 < 3 ÁL2) 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 Horvßth 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 Ám 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-Ám SPP with a shell thickness of 0.25 Ám.
In 2007, a revolution started with the commercialization of a new generation of sub-3-Ám SPPs adapted for separation of small
and large molecules (7) by Advanced Materials Technology. This material possesses a 1.7-Ám solid core covered by a 0.5-Ám-thick
shell of porous silica. It combined the advantages of both fully porous and nonporous particles. In particular, this improved
particle design solved the problem of the low loading capacity of early pellicular particles because an approximately 75%
volume fraction of these particles is still porous. Since then, many providers commercialized SPPs with particle sizes ranging
between 1.3 and 5 Ám. Table I provides a current list of commercially available columns packed with SPP materials, their pertinent
properties, and available bonded phases.
Table I: A current list of HPLC columns made with commercially available SPP materials including the newest sub-2-Ám SPPs