Molecular weight analysis of polycations by capillary electrophoresis in a solution of neutral polymers

Under appropriate conditions, polyelectrolytes separate according to molecular weight during their electrophoretic migration through a dilute solution of inert neutral polymers. These separations facilitate a new capillary electrophoresis-based approach for high resolution and high throughput polyelectrolyte molecular weight analysis, one theoretically applicable to both polyanions and polycations. Although pioneered for DNA, a polyanion, the new method is discussed here in the context of synthetic polycations. Numerous experimental difficulties evolve from the introduction of positive solute charge, not the least of which is a strong tendency for solute adsorption on the negative capillary walls. The adsorption can be overcome by using a run buffer with a cationic surfactant that forms a dynamic yet stable positive wall coating. Feasible at nearly any pH, the surfactant approach enables robust and high-resolution polycation analysis. For illustration, we compare the separation at low pH of three protonated poly(2-vinylpyridine)s to the separation at neutral pH of the same polymers after quaternization. A good match is found. Further, electrophoretic analyses by the new method suggest how poly(2-vinylpyridine)s degrade/crosslink when exposed to quaternizing conditions for an excessive period.