Measurement of the eddy dispersion term in chromatographic columns. II. Application to new prototypes of 2.3 and 3.2 mm I.D. monolithic silica columns

The mass transfer mechanisms in silica monolithic columns of the second generation were investigated, using four research samples (two 2.3 mm × 50 mm and two 3.2 mm × 50 mm silica rods) provided by their manufacturer. The heights equivalent to a theoretical plate (HETP) of these columns were measured in a range of mobile phase velocities, following a meticulous experimental protocol. The coefficients of the van Deemter equation (longitudinal diffusion term B/uS, skeleton/eluent mass transfer resistance term CuS, and eddy diffusion term A) were determined. The protocol includes using the peak parking method (to determine the longitudinal diffusion term), an accurate model of effective diffusion in silica monolithic structures (to determine the skeleton/eluent mass transfer resistance term), and an accurate method to measure the column HETP and determine the eddy diffusion term. The results show that the minimum plate heights of these new monolithic columns ranges between 4 and 5 μm, three to four times lower than those observed for monolithic columns of the first generation. A detailed analysis of the eddy diffusion term demonstrates that this improvement in column efficiency is partly explained by the reduction of the domain size (the sum of the skeleton and throughpore sizes, −40%) but mostly by an increase of the radial homogeneity of the monolithic rods. The columns of this second generation exhibit residual trans-column relative velocity biases as low a 1.4% (instead of 3% for previous columns), a value which is comparable to those observed in 4.6 mm I.D. columns packed with sub-3 μm core–shell particles, with which they might become competitive.