Change in conductivity in non-cross-linked polyacrylamide capillary electrophoresis - Effects of aging polyacrylamide and buffer composition

Upon application of an electric field to a polymer-filled capillary, there is a rapid expulsion of excess ions associated with the polymerization mixture. This expulsion is complete in about 10-15 min. A much slower transport of ions occurs at one end of the capillary; the transport arises because of the difference in transference number between the buffer reservoir and the polymer filled capillary. This ionic transport leads to a change in the conductivity of the polymer at one end of the capillary. We have monitored the conductivity profile in 5% T non-cross-linked polyacrylamide capillaries at 300 V/cm for different ages of polyacrylamide. The profile was fit with a double-exponential, corresponding to the rapid expulsion of reaction ions and the slow transport of ions due to differences in transference number. The fit indicates that there are more ions in polyacrylamide as it is aging, and that the depletion of ionic concentrations is less important in aged polyacrylamide than in fresh polyacrylamide. The effect of the buffer composition was studied. By changing from 1XTBE buffer reservoirs to a mixture of 30 mM KCl and 1XTB buffer, the depletion of ionic concentration at the injection end disappears and is replaced by an increase of ionic concentration at the ground end. By changing 1XTBE buffer reservoirs to 1XTBE, 5% T non-cross-linked polyacrylamide and seven molar urea buffer reservoirs the depletion of ionic concentration at the injection end disappears.