Comparison between the efficiencies of columns packed with fully and partially porous C18-bonded silica materials

The chromatographic performance of a new brand of shell particles is compared to that of a conventional brand of totally porous silica particles having a similar size. The new material (Halo, Advanced Materials Technology, Wilmington, DE) is made of 2.7 μm particles that consist in a 1.7 μm solid core covered with a 0.5 μm thick shell of porous silica. The other material consists of the porous particles of a conventional 3 μm commercial silica-B material. These two columns have the same dimensions, 150 mm × 4.6 mm. The reduced plate heights of two low molecular weight compounds, naphthalene and anthracene, two peptides (lys-bradykinin and bradykinin), and four proteins, insulin, lysozyme, β-lactoglobulin, and bovine serum albumin were measured in a wide flow rate range and analyzed on the basis of the Van Deemter equation and of modern models for its terms. The Halo column provides a smaller axial diffusion coefficient B and a smaller eddy dispersion term A than the other column, a result consistent with its lower internal porosity (∈p = 0.19 versus 0.42) and with the narrower size distribution of its particles (σ = 5% versus 13%). The two columns have similar C terms for the two low molecular weight compounds and for the two peptides. However, the C term of the proteins that are not excluded is markedly lower on the column packed with the Halo particles than on the other column. A recent theoretical analysis of the mass transfer kinetics in shell particles predicts a C term for moderately retained proteins (3 < k′ < 5) that is about 35% lower for shell than for fully porous particles while the experimental data show a value nearly 45% lower, an excellent agreement considering that the internal tortuosity of the particles might be different, affecting the ratio of the effective diffusivities (Deff) of the proteins in the two materials. Surprisingly, the Kozeny–Carman constant of the Halo packed column is 50% larger than that of the other column, in spite of which the permeability of the Halo column is slightly larger, due to its larger external porosity.