The Mass Transfer Mechanism of Columns Packed With sub-3μm Shell Particles and its Reproducibility for Low- and High-Molecular Weight Compounds

The mass transfer mechanism and the column-to-column reproducibility of the efficiency of 2.1 and 4.6mm x 100mm columns packed with the same batch of 2.7μm Poroshell120-C18 core-shell particles was investigated. The accurate measurement of the band broadening phenomena provided unambiguous explanations for their exceptional kinetic performance. With respect to columns packed with fully porous particles, the gain of efficiency observed for small molecules was due to a 40% and 30% reduction of the trans-column eddy dispersion and longitudinal diffusion HETP terms, respectively. For larger molecules, it was explained by the 40% reduction of the solid-liquid mass transfer resistance HETP term. The 25% smaller efficiency of the 2.1 versus the 4.6mm I.D. columns stemmed from the increasing contribution of the trans-column eddy dispersion (wall effects) to the overall eddy dispersion with decreasing the column I.D. Relative standard deviations of 8% were found for the distribution of the optimum HETPs of these 2.1 mm and 4.6mm I.D. columns, respectively. 

The concept of shell, core-shell, or superficially porous particles is not new. Some fifty years ago, the terminology of pellicular stationary phases was employed by Cs. Horvath when he and his co-workers prepared some new ion-exchange columns designed for the HPLC separations of highmolecular weight compounds of biological interest [1]. They suggested the idea of coating 50μm glass beads (impermeable cores) with a thin layer of anion-active resin. On the same line, Knox recommended the use of thin films of liquid stationary phase in liquid-liquid chromatography [2] and Parish proposed preparing a shallow surface layer of ion-exchange groups around cross-linked polystyrene beads for the separation of metal ions [3]. The incentive for the preparation of such new stationary phase architectures was clear: decrease the solidliquid mass transfer resistances by decreasing the average diffusion distance of the analytes across the stationary phase volume.

Back then, the average particle sizes were as large as 50 to 100μm, e.g., so that the column efficiencies were essentially dictated by the mass transfer resistance between the particle volume and the mobile phase. Reducing the porous layer thickness led then to a significant improvement in the resolution of complex mixtures [4-6]. However, over the next forty years, the competition of packing materials made with high-purity, fine fully porous particles increased ceaselessly over time [7]. Additionally, pellicular particles have a much lower loading capacity than that of fully porous particles so the injected sample concentrations should be kept to a minimum in order to avoid column overloading and nefarious peak distortion. So, pellicular particles encountered little commercial success (50μm Corasil I and II, Zipax, Pellicosil [8, 9], and 5μm Poroshell [10]) and fell into oblivion until, in 2007, they were resuscitated as the modern sub-3 μm Halo shell particles [11] (Advanced Material Technologies, Inc). Most remarkably, these sub-3μm shell particles provided column efficiencies comparable to those achieved with sub-2μm fully porous particles [12] with the advantage of operating at about twice lower back pressure [13]. So, it became possible to achieve ultra-high column performance using the standard 400 bar instruments, provided that some low cost modifications (smaller I.D. connectors, smaller detection cell volume) be made to these instrument [14,15]. 

Rapidly, new brands of sub-3μm superficially porous particles emerged on the market with the 2.7μm Ascentis Express (Supelco, Inc), 2.6μm Kinetex (Phenomenex, Inc), 2.7μm Poroshell120 (Agilent Technologies, Inc), 2.6μm Accucore (Thermo Scientific, Inc), 2.7μm Nucleoshell (Macherey-Nagel, Inc), and 2.6μm Sunshell (Chromanik, Inc). The names of shell, core-shell, or superficially porous particles replaced the old terminology “pellicular particles” because of the large thickness of the porous layer relatively to the particle diameter. All these particles were made of a 1.6 to 1.9μm solid silica cores, covered with a 0.35 to 0.50μm thick porous shell. Such particle architecture solved the issue of the low loading capacity since 60 to 75% of their volume versus only less than 15% for the old pellicular particles is now accessible to the analytes [16]. As a result of their increasing success, five main questions/debates surged about the exceptional kinetic performance of columns packed with shell particles: (1) Which mass transfer mechanism can properly justify why columns packed with shell particles provide lower reduced plate heights than those packed with conventional fully porous particles?

Based on widespread chromatographic beliefs, most of the literature reports and all the advertising brochures suggested hastily that this was due to the reduction of the diffusion path across the stationary phase and to the narrow size distribution of this new packing material [11, 49]. This has never been proven experimentally and theoretically, especially for small molecules. (2) What is the columnto- column reproducibility of such highperformance columns? No data are yet available in the literature on this topic. They are critical for both the manufacturers, who want to minimise their rebuttal levels, and for.....Read the full Article