Theoretical examination of electroosmosis control with external radial electric field in capillary electrophoresis.

The influence of an external radial electric field (E(R)) on electroosmosis in capillary electrophoresis was studied theoretically. Based on a Stern-like model, three basic equations were deduced, with only two unknown parameters of delta and psi(d) where delta is the distance between the flowing shear interface and the tube wall, while psi(d) is the potential at the starting point of diffuse layer. The new equations reveal that, to effectively regulate the electroosmosis at E(R) <3 x 10(8) V/m, buffer pH should be kept below 5. In a common case of E(R) <10(8) V/m, the buffer pH should be below 4, otherwise the flow direction of the electroosmosis cannot be reversed. A way to increase the working pH range lies in the use of chemically coated rather than bare tubes. As expected, small capillaries and low ionic strength are preferred.