About the C term in the van Deemter's equation of plate height in monoliths.

Monolithic stationary phases provide high rates of mass transfer at low pressure drops and enable much faster separations. Experimental data from published H versus u plots obtained with monolithic columns are compiled with an emphasis on the mass tranfer (C term of the van Deemter equation). Silica-based monoliths of the C18 Chromolith type exhibit a very low C value that does not vary too much within a wide range of flow rates. The C term is a function of retention, temperature, molecular weight of the solute and mesopore size. Silica-based monoliths of capillary format exhibit a slightly higher C value which slightly increases with increasing flow rates. The key parameter is the through-pore size/skeleton size ratio. Experimental data on polymer-based monoliths retrieved from the literature mostly concern acrylate-based material in a capillary format. Due to the unique character of every column the scatter of published C values is wide. The observed C term are higher than those observed with silica monoliths and increase quite linearly with the flow rate. Low-density polymers exhibit better performances than high-density polymers. The majority of HETP versus u plots are drawn from the peak width at half height. Accurate data should be acquired through the second moment of the peak. Some equations for C have been derived. An equivalent dispersion particle diameter is a measure of band broadening in the mesopores. Mass transfer in silica monoliths is well accounted by the theory developed by Guiochon et al.