OFF-COLUMN DETECTION WITH PRESSURE COMPENSATION IN CAPILLARY ELECTROPHORESIS

A theoretical description has been developed of the zone broadening in capillary electrophoresis when the electric field is decoupled before the detector. With off-column detection, the parabolic flow profiles induced by the electroosmotic flow contribute to zone broadening in the separation and the coupling capillary. Good agreement is found between the theory and the experimental data on zone broadening with on-column detection. With off-column detection a zone-broadening contribution equivalent to approximately 15 nL is not accounted for by the theory. This contribution may be attributed to the field decoupler. It is shown that zone broadening can be decreased to a large extent by applying a compensating pressure on the front end of the separation capillary during electrophoresis. Optimization of the coupling-capillary dimensions, the field strength, and the applied pressure is discussed. Under appropriate experimental conditions, plate numbers up to 160 000 have been obtained with off-column detection.