Cyclic capillary electrophoresis.

A strategy is described here for increasing both the resolution and the flexibility of capillary electrophoresis performed in a sieving medium of ungelled polymer. This strategy is based on analysis and, sometimes, re-analysis that is done in several stages of constant-field electrophoresis. Enhancement-stages are between the analysis-stages. An enhancement-stage (i) increases the separation between peaks, while (ii) moving DNA molecules in the reverse direction. An enhancement-stage is based on an electrophoretic ratchet generated by a pulsed electrical field that can be zero-integrated. The ratchet-generating pulses are longer than the field pulses that have previously been used to improve the resolution of DNA molecules. No limit has been found to the resolution enhancement achievable. Apparently, diffusion-induced peak broadening is inhibited and, in some cases, may be reversed by the ratchet. The enhancement-stages are critically dependent on the electrical field-dependence of a plot of electrophoretic mobility as a function of DNA length. To generate the pulsed electrical field, a computer-controlled system with a time resolution of 30 microseconds has been developed. Programming is flexible enough to embed other pulses within ratchet-generating pulses. These other pulses can be either the previously used, shorter field-inversion pulses or high-frequency periodic oscillations previously found to sharpen peaks.