On the optimization of the shell thickness of superficially porous particles

The thickness of the porous shells of superficially porous particles influences the separation power of columns packed with these packing materials. Models of the mass transfer kinetics across porous adsorbents permit the prediction of the HETP curves of columns packed with particles having shells of different thicknesses, for molecules of different sizes. Decreasing the thickness of the porous layer potentially results in lower values of the "C-term" of the HETP curve and of the minimum of these curves. The Poppe plots calculated under isocratic and gradient conditions show that the separation power of columns packed with superficially porous particles increases significantly with decreasing thickness of the porous layer but this increase is more important for larger than for smaller molecules. The resolution between pairs of compounds increases at constant values of their retention factors when the strength of the eluent must be reduced to compensate for the decrease of their retention that is caused by the reduction of the surface area of the stationary phase. Thus, the separation power of columns packed with superficially porous particles increases with decreasing shell thickness. In contrast, if analysts do not compensate for the retention decrease, the resolution between small molecular weight compounds becomes worse with thin than with thick superficially porous particles. Finally, the importance of using instruments providing low extra-column band broadening contributions is stressed.