Recent Trends and Advances in Superficially Porous Particle Technology:  Application to Large Molecule Separations

Superficially porous particles have become a major platform for the development of high performance liquid chromatography columns.  Since their modern introduction in 2006, most major column manufacturers have adopted the technology.  A number of particle sizes and a variety of chemical modifications are now available.  In more recent years, the technology has been adopted for larger molecule separations.  This contribution aims to provide some highlights of these recent trends.

Introduction:

Over the past decade there has been a concerted effort to increase separation efficiency in high performance liquid chromatography (HPLC).  The benefits of increasing efficiency include faster analysis times and enhanced resolution.  The trend began with the development of smaller particles (generally termed sub-2µm) and the instrumentation advances required to handle the resultant high-pressure requirements, dawning the age of ultra-high performance liquid chromatography (UHPLC).  UHPLC using sub-2µm particles has gained widespread use in many industries; however, the financial burden of purchasing new instrumentation has hindered adoption by many potential users.  The modern introduction of superficially porous particles (SPP) in 2006 (Halo, Advanced Materials Technology) provided a means of attaining high separation efficiencies with less backpressure burden.  The lower backpressure afforded by the SPP architecture has allowed users of both traditional HPLC systems and UHPLC alike to realise high efficiency separations.

There have been many reviews published regarding SPP properties, theoretical treatments and uses over the past several years [1-3]. This contribution is intended to provide a review of some recent trends in the development of SPP based columns, namely the adaptation of SPP technology towards large molecule separations. 

Advantages of Superficially Porous Particles

Superficially porous particles or solid core, core-shell and Fused-Core®, as they are often termed, are characterised by having a solid core (typically nonporous silica) surrounded by several layers of porous silica.  The first modern SPP phase was introduced by Advanced Materials Technology (HALO) in 2006.  The particle design provides a number of advantages over fully porous materials.  Peak broadening is generally modelled using the van Deemter equation (H = A + B/µ + Cµ) where A represents eddy diffusion, B; axial diffusion and C; mass transfer effects.  The shorter diffusion path within the ‘working’ porous shell yields improved mass transfer kinetics over fully porous materials (lowering the C term) allowing higher flow rates to be utilised without significantly deteriorating peak efficiency.  This effect is most notable for large molecules that exhibit slower diffusion constants.  Secondly, the inherent small particle size distribution resulting from the construction process has been attributed to increasing the quality of column packing homogeneity [4].  This may lead to lessening peak broadening due to eddy diffusion.  Lastly, the greater permeability of SPP over similar sized fully porous particles allows for higher flow rates and thus effectively reduces axial diffusion that contributes to band-broadening.  Although there is still some debate regarding the entire fundamental reasons SPP provide increased efficiency over fully porous particles, it is clear that greater efficiency with respect to similarly sized fully porous particles is realised.

Since the release of HALO columns to the market, most of the major column manufacturers have adopted some form of SPP technology.  A listing of major brands and their offerings are provided in Table 1.  Since the onset of SPP adoption, the manufacturers have added many of the classical HPLC surface chemistry modifications such as C8, cyano and aromatic phases.  
Most manufactures have also added different overall particle sizes and, in some cases, pore sizes to accommodate the increased breadth of applications.  The initial columns released were primarily packed with 2.6 and 2.7 µm 
SPP particles with pore sizes around 100 Å. Larger pore sizes (150 – 200 Å) that allowed separations of polypeptides were furthermore introduced [5].  Sub 2-µm versions of SPP based columns have been developed that combine both the efficiency advantages of the smaller particle with the architecture of the core-shell.  Additionally, 5 µm SPP sizes have been developed by several manufacturers.  These larger particles allow more direct transfer of classical 5 µm applications, such as USP methods, to the newer technology.  Most recently, the focus in the industry has been moving toward the adaptation of SPP technology for large molecule separations.  

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