Two separation mechanisms are demonstrated for DNA fragments in free solution inside narrow capillary tubes. No sieving medium or buffer additive is needed. In the electromigration mode, a likely model is that dissociated silanol groups on the capillary walls create an excess of positive charges in the diffuse layer near the surface. These provide electrostatic shielding of the negatively charged DNA fragments to lower their electrophoretic mobility against electroosmotic flow. Small fragments can access the diffuse layer more readily because of lower steric hindrance and larger diffusion coefficients and are therefore eluted earlier than the large fragments. In the hydrodynamic mode, the Poisseuille flow profile causes molecules near the surface to move slower. So, the smaller DNA fragments are eluted later than the larger fragments.
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