Effects of Particle Porosity on the Separation of Larger Molecules

The pore structure of a chromatographic stationary phase accounts for the vast majority of the surface area responsible for the separation. The pore diameter influences the overall surface area and with it the capacity of the column but also limits the size of the analyte the column can or should be used for. In adsorption chromatography the limiting effect of the pore diameter is further enhanced by adsorbed analyte molecules partially blocking the pore structure. In the analysis of large molecules pore diffusion becomes a crucial parameter in the efficiency of the column. A number of models deal with hindered mass transfer in porous systems. Effects of particle porosity on the separation of larger molecules are discussed and examples for the separation of small, medium and large analytes on various pore size stationary phases are given. Pore size is shown to be an important parameter when analyzing larger molecules. By selecting the right pore size for a task the capacity and the mass transfer trade-off can be optimized to achieve the best possible separation. For tryptic digests a 200 A stationary phase is superior to a 300 A due to its two-fold increase in surface area while 1000 A are required for the separation of proteins.