Electroosmosis and electrochromatography in narrow bore packed capillaries
Electroosmosis, the electrically induced flow of liquid, can be used as an alternative to pressure driven flow in liquid chromatography. Consideration of the physical nature of this type of flow suggests that less peak dispersion should result than with conventional flow. In addition theoretical considerations indicate that it should be possible to work with particles far smaller than those currently used in high performance liquid chromatography. This work describes the use of electroosmosis to propel electrolyte through narrow capillaries packed with typical chromatographic packing materials. The resistive heating generated by the passage of electric current through the medium dictates that this must be carried out in capillaries of less than 200 µm i.d. The experimental methods used with such capillaries, including the production of packed capillaries are discussed. Experiments carried out in capillaries packed with particles as small as 1.5 µm in diameter demonstrate that adequate linear flow velocities can be obtained with particles which are too small for use in conventional chromatography due to pressure limitations. Measurements of the linear velocity of electroosmotic flow show that there is no evidence of a relationship between linear velocity and particle diameter, for particles ranging from 1.5 µm to 50 µm in diameter. The direct comparison of plate heights obtained using both pressure driven flow and electroosmotic flow shows’ that in the latter case considerably enhanced efficiencies are obtained. In some cases reduced plates heights as low as 0.7 have been achieved, providing strong evidence of a negligible contribution to the overall plate height from the van Deemter A-term. Using 1.5 µm diameter particles plate numbers of 300,000 have been obtained for an unretained analyte in a time of approximately 5 minutes. The implications of the results together with the performance limitations of electrically driven chromatography (electrochromatography) are discussed.