Determination of electroosmotic flow mobility with a pressure-mediated dual-ion technique for capillary...

A method is described for the indirect determination of the mobility of the electroosmotic flow (EOF), which can be carried out within a few minutes even for very low mobilities. It is independent of the direction of the EOF. It is based on the comparison of the measured mobilities of two oppositely charged reference ions (tetraphenylphosphonium and tetraphenylborate) with given mobilities in different organic solvents (methanol, acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, propylene carbonate) at ionic strengths between 5 and 50 mM. The method is based on the sequential movement of the reference ions in a three-step process: first by a laminar flow to a certain position in the separation capillary, followed by electromigration due to application of voltage, and pressurised migration towards the detector. In this way the total mobilities of the reference ions can be determined from their residence times, and the difference to their known actual mobilities gives the mobility of the EOF. The method avoids misinterpretations caused by system- and eigen-peaks, which often bias the results especially when a conductivity detector is used. The method is suitable for all solvents, and is an advantage especially for organic and mixed aqueous-organic background electrolytes with high UV absorbance.