Using NMR displacement imaging to characterize electroosmotic flow in porous media

Pulsed field gradient nuclear magnetic resonance (PFG-NMR) and NMR imaging were used to study temporal and spacial domains of an electrokinetically-dr iven mobile phase through open and packed segments of capillaries. Characte ristics like velocity distribution and an asymptotic dispersion are contras ted to viscous flow behavior. We show that electroosmotic flow in microchan nel geometries can offer a significant performance advantage over the press ure-driven flows at comparable Peclet numbers, indicating that velocity ext remes in the pore space of open tubes and packed beds are drastically reduc ed. An inherent problem of capillary electrochromatography that we finally address is the existence of wall effects when in the general case the surfa ce zeta-potentials of the capillary inner wall and the adsorbent particles are different. Using dynamic NMR microscopy we were able resolve this syste matic velocity inequality of the flow pattern which strongly influences axi al dispersion and may be responsible for long time-tails of velocity distri bution in the mobile phase. (C) 2001 Elsevier Science Inc. All rights reser ved.