Size-exclusion electrochromatography with controlled pore flow

In size-exclusion electrochromatography (SEEC) there exists an optimum in p ore-to-interstitial flow ratio with respect to the resolution. Two methods for finding and controlling the optimal pore-to-interstitial flow ratio in SEEC have been studied: (i) varying the ionic strength of the mobile phase and (ii) the application of a hydrodynamic flow in addition to the electrco -driven flow. Both methodologies have been evaluated in terms of efficiency and applicability with columns packed with silica particles containing por es of either 10 or 50 nm in diameter, and with different ionic strength mob ile phases. Using the first method with the 10-nm pore particles, the how r atio could be adjusted within an appropriate range. However, with the wide- pore (50 nm) particles it appeared that the pore-to-interstitial flow ratio was too high at all conditions tested to obtain proper selectivity. In the second approach, the desired pore flow was generated by the electric field and the pore-to-interstitial flow ratio could then be adjusted by an appli ed pressure over the column. This method was applicable with both particle types studied. The application of a (low) voltage gradient in addition to a pressure-driven flow resulted in a sharply improved separation efficiency as a result of a strongly improved mass transfer due to intra-particle elec troosmotic flow. When optimized, pressurized SEEC generates identical separ ation efficiencies for polystyrene standards as are obtained with pure SEEC , while the reduction in selectivity, in comparison to pressure-driven SEC, is kept minimal. (C) 2001 Elsevier Science B.V. All rights reserved.