Extremely high electric field strengths in non-aqueous capillary electrophoresis.

The influence of high electric field strength on the separation of basic analytes in non-aqueous alcohol background electrolyte (BGE) solutions was investigated. Increasing the separation voltage in capillary electrophoresis (CE) may be advantageous if the conductivity of the BGE solution is low enough to allow fast separations without excessive Joule heating or band broadening. The voltage range tested was 20-60 kV with methanol and ethanol, and 25-60 kV with propanol and butanol as solvent for BGE. The resulting electric field strengths ranged from 660 V cm(-1) to 2000 V cm(-1). Experiments were made with a special laboratory constructed CE instrument. The separation efficiency vs. voltage curve was found to vary with the alcohol BGE solution. The increase in voltage decreased the separation efficiency in the case of methanol BGE solution, but with the other BGEs a clear efficiency maximum was obtained above 30 kV. The highest separation efficiencies were achieved with propanol BGE solution, where the efficiency maximum was reached at 45 kV. However, reasonable efficiency was achieved even at 60 kV. The extent of Joule heating was determined by calculating the temperature inside the capillary and the observed plate heights were interpreted in terms of the Van Deemter equation. The decrease in the separation efficiency with higher voltage was attributed mainly to Joule heating in the case of methanol and ethanol BGE solution and to the analyte adsorption on the capillary wall with propanol and butanol BGE solutions.

Compensation of the siphoning effect in nonaqueous capillary electrophoresis by vial lifting.

Increasing the sample load in nonaqueous capillary electrophoresis through the use of wide-bore capillaries is a good way to scale up analytical separations to semipreparative level. However, obtaining high efficiency requires the use of special instrumentation to eliminate siphoning. When wide-bore capillaries are employed, relatively large solvent volumes are transported from inlet to outlet vial, and due to the difference in liquid levels a siphoning flow from outlet to inlet is established. Siphoning induces a deviation from the plug-like flow profile and adversely affects the separation efficiency. In this study the use of wide-bore capillaries in nonaqueous capillary electrophoresis was examined with compensation for siphoning by lifting of the inlet vial. The inlet vial is raised at a speed appropriate for maintaining equal levels of liquid in the inlet and outlet vials. The optimal lift rate was determined empirically from a series of runs in which the lift rate was varied. As well, a simple theoretical model was devised for the calculation of lift rates. The model was successfully applied for the 200 microm and 320 microm ID capillaries but for the 530 microm ID capillary the predicted optimal lift rate was too low. Evidently this was because the theory was unable to account for the effect of siphoning on the migration times. Three model compounds, bumetanide, furosemide and ethacrynic acid, were separated using an acetonitrile-ethanol mixture (50:50, v/v) with potassium acetate (1 mM) or ammonium acetate (5 mM) as electrolyte. Good separation of bumetadine and ethacrynic acid was obtained even with a 530 microm ID capillary when the lift rate was carefully optimized. Without elimination of siphoning the peaks would not have been detectable. The viscosities and electrical conductivities of the electrolyte solution measured at different temperatures showed that viscosity as well as conductivity decreased with increasing temperature. The temperature dependence of the conductivity was used to estimate the temperature inside the CE capillary.