A simple model for the optimization of peak variance In sample stacking with a gravity-injected sample has been developed. In sample stacking, a long plug of low-concentration buffer containing a sample for separation Is Introduced Into a column filled with a buffer of the same composition but of a higher concentration. The sample Ions then migrate very rapidly under the enhanced applied electric field In the sample plug until they reach the concentration boundary. Once they cross the boundary, the Ions slow down and stack Into a narrow band. Theoretically, the peak width In sample stacking Is proportional to the ratio, -y, of buffer concentration in the original sample solution to that In the column. However, this difference In the concentrations Inside the capillary column also generates an electrooemotlc pressure originating at the concentration boundary. The laminar flow resulting from the electrooemotlc pressure causes extra peak broadening. Sample stacking and laminar broadening work against each other to yield an optimal point relating to the sample buffer concentration, the separation buffer concentration, and the sample plug length. The predications of the model are In agreement with experimental results.

Sampk dacklng k a rlmple ontolumn concentration technlque In hlgh-perfonnance capillary electrophorosk (HPCE).However, the amount d r(mpk whlch can be loaded Into the column In conventbnal sample dacklng k rather Ibnlted becaw0 of dktubrrncer cauwd by the lowconcentration sample buffer. This report describes a technique d dacklng an extremely large sample volume Into narrow bands. The technique k baeed on the prlnclpal that the local oiectrophoretlc vdodty of the Ions lndde the sample Mer k much raster than the bulk e k c t r m o t l c velocity d the rolutkn. By applyhg a voltage wlth rovemod pdarlty Inmodlately after kadlng the sample, one can remove the sample buffer prlor to separation of anaiytes. Slnce the peak-broadenlng mechanbm b removed from the system, better mo&Iion can be achbved for a large sampk vokme. S h e large amounts of ample can bo loaded onto the cokmn, a factor of several hundreds in rlgnal enhancement Is obtalnod.

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Electroosmotic flow In a fused-slllca capillary column, partially filled with a buffer of one concentration and containing a second buffer of the same composition but different concentration, Is studied. The bulk electroosmotic velocity In this kind of mixed buffer system Is derived and shown to be a weighted average of the electroosmotic velocities of the pure buffers. The theory of laminar flow caused by the mismatch between electroosmotic velocities Is developed and shown to cause extra peak broadening for samples Inside the column. Good agreement with experimental results Is found. The length of new buffer Introduced by electroinjection can be determined from the variation In electrophoretic current during Injection.

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