Effect of hydrostatic flow on the efficiency in capillary electrophoresis

Abstract

An expression is derived for the plate height in capillary electrophoresis for cases where hydrostatic flow (HF) is present. HF will occur whenever the buffer levels at both ends of the capillary are not at the same height. Whereas the plate height equation for an HF-free system has only a molecular diffusion term, the plate equation in the presence of HF has an additional term which is a resistance to mass transfer term. The second term is a function of capillary radius, the hydrostatic velocity, the solute diffusion coefficient and the electrophoretic velocity. Unlike in chromatography, the mass transfer term usually does not increase with increasing solute velocity. Nonetheless, the contribution of this additional term to the total plate height can be substantial for wide capillaries and large solute molecules. We calculated maximum allowable buffer height differences, Δ h max, for a given loss in plate height. It was found that for large molecules and wide capillaries, Δ h max, can be less than 1 mm, making severe demands on the instrumental design. With small solutes and narrow capillaries, the requirement for extract buffer levelling at both ends of the capillary is less acute.