electroosmotic flow in conventional CZE. Equations 22 and 23 show that the effective sample plug length using field-amplified sample injection is smaller than the plug length using conventional electroinjection because of the stacking effect. Consequently, one can better stack the sample under the enhanced field by injecting for a much longer time or at higher voltages without much peak broadening from the longer sample. Figure 6 shows a typical improvement in the signal by using fieldamplified sample injection compared with conventional electroinjection. In theory, a maximum signal enhancement is achieved when one switches the column directly from the highconductivity support buffer to the low-conductivity sample solution. However, manipulation of the column during the injection process can produce a physical disturbance at the end of the capillary, which causes improper field amplification at the injection point. In addition, the sample ions will stack at the injection point and cause a degradation of the field enhancement if there is no low-
concentration buffer at the injection end. More importantly, an initial plug of low-concentration buffer is needed for field-amplified sample injection of negative ions under r e versed polarity. Polarity-switching sample inj e c t i o n . In the absence of electroosmotic flow, one can apply fieldamplified sample injection to achieve sample enhancement for either positive or negative ions by choosing the proper polarity of t h e electrodes. However, if there is a large electroosmotic flow, switching the column directly into low-concentration sample solution will only inject ions that migrate in the same direction as the bulk flow. Under normal polarity, where the electroosmotic flow is toward the outlet of the column, the high field strength at the injection point will push away ions that have negative mobilities with respect to the electroosmotic flow. With r e versed polarity, although the high field strength will pull the negative ions into the column, they will be carried out immediately from the injection end by the electroosmotic flow during sample injection.
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However, as we mentioned earlier, the bulk electroosmotic velocity of the solution is much slower than the electrophoretic velocity of sample i o n s u n d e r t h e e n h a n c e d field. Therefore, by introducing a short plug of diluted buffer before sample injection and reversing the polarity of the electrodes, we can inject and concentrate negative ions deep inside the capillary column. A subtle difference in injecting the negative ions is that the diluted sample buffer is now moving out of the column from the injection end. C o n s e q u e n t l y , t h e length of the diluted sample buffer should be long enough so that some of it remains inside the column by the end of the sample injection time. The positive ions will now be pushed away from the column during injection under the reversed polarity. A very effective charge discrimination s i g n a l - e n h a n c e d injection t h u s is achieved. After sample injection, the column is switched back to high-concentration support buffer and the separation voltage with the normal polarity is applied to the column. The bulk solution and the sample plug now mi-
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