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axial molecular diffusion presents the ultimate limitation toperformance in CE (for an unretained solute), and the plate heightbecomesh a ) 2D m (T)/d c u eo (14)Temperature effects were quantitatively characterized and canexplain, via the temperature dependence of the molecular diffusioncoefficient, the increasing variance of the Gaussian axial displacementprobability distributions recorded at higher electroosmoticflow velocities (and associated levels of heat dissipation in thecapillary).The low values of reduced plate height observed with boththe CHPLC and CEC modality (h a < 1) demonstrate the excellentperformance of the underlying fluid flow field in the packed bedregion, even with an aspect ratio of only 6.25. Compared to thepressure-driven mobile phase (h min ) 0.83), however, the electroosmoticflow shows a still superior efficiency (h min ) 0.55). It isattributed to the improved flow uniformity on any time and lengthscale over the whole column cross section and to the differencein the hydrodynamically effective particle diameter concerning thetotal fraction of stagnant fluid in the column and associated,diffusion-limited mass transfer. These effects, which affect thea- and c-terms in the Knox equation but leave unchanged theb-term, also explain the shift of the minimum of the CEC plateheight curve to higher reduced flow velocities.ACKNOWLEDGMENTU.T. gratefully acknowledges the award of a Marie CurieFellowship under the Training and Mobility of Researchers (TMR)Program from the European Union (Contract ERBFMBI-CT98-3437). This research was further supported by the EuropeanCommunity activity Large-Scale Facility Wageningen NMR Centre(Contract ERBCHGE-CT95-0066) and by the Technology FoundationSTW (Project WBI.3493), Applied Science Division of theDutch Science Foundation NWO.Received for review November 15, 1999. AcceptedFebruary 22, 2000.AC991303IAnalytical Chemistry, Vol. 72, No. 10, May 15, 2000 2301