The principles of migration and dispersion in capillary zone electrophoresis in nonaqueous solvents

Abstract

Nonaqueous solvents are interesting media for capillary zone electrophoresis as they can affect all relevant parameters governing the separation of sample zones. However, for a rational planning of the working conditions and an appropriate interpretation of the results obtained, the basic principles of ion migration and zone dispersion must be understood. Many solvent induced effects need to be carefully considered and recognized before full exploitation of nonaqueous solvents can take place. It is the goal of this overview to present the fundamental physicochemical aspects of capillary zone electrophoresis in nonaqueous solvent systems. Therefore, the detailed discussion is related to the effect of organic solvents on electrophoretic mobilities (based on the theory of conductance), acid-base dissociation behavior (based on the transfer activity coefficient and medium effect), pH, separation efficiency (with regard to mobility and diffusion coefficient in dilute solutions), resolution, and electroosmotic flow.

Are the asserted advantages of organic solvents in capillary electrophoresis real? A critical discussion

“…This pH is named point-of-zero-charge; in water it is reached at very low pH (,1.5). This point is also shifted to higher values in organic solvents, e.g., in MeOH to pH around 7 [14,25] (see Fig. 6).…”

“…Thus, the relative differences of the mobilities in a given solvent do not change so much, and differences in the actual mobilities are comparable to those of the absolute mobilities. Note, however, that the absolute variation of the mobility as function of the ionic strength can differ much stronger among the solvents (see, e.g., [10,13,14]). An example is given in Fig.…”

“…We took not only the absolute mobilities, which we found in the literature, but also the respective actual mobilities calculated according to the equation of Robinson and Stokes [18] (converted for mobilities as in Ref. [14]). The averages and spreads (SDs) of the selectivity coefficients derived in this way at 10 mmol/L ionic strength are shown in Fig.…”

“…2 with the aid of conductivity equation of Robinson and Stokes [18] (modified for mobilities in Ref. [14]). Ion size parameter 5 Å was used in all calculations.…”

“…This factor is always smaller than unity, which is an outcome of the stronger reduction of the mobility with ionic strength compared to the respective reduction of diffusion coefficient. The mobility is reduced by both the relaxation effect and the electrophoretic effect, whereas diffusion is retained only by the relaxation effect (for detailed discussion, see [14,32]). A consequence is that the plate number is always smaller at finite ionic strength than at zero ionic strength.…”