pH-mediated field amplification on-column preconcentration of anions in physiological samples for capillary electrophoresis.

Two limitations of capillary electrophoresis (CE) are the low sample loadability of the capillary and an incompatibility with high ionic strength samples. Several strategies have been described to preconcentrate and lower the ionic strength of physiological samples prior to CE analysis. These have included both off-capillary and on-capillary approaches. We have previously described a version of on-column field-amplification stacking termed pH-mediated stacking. pH-mediated stacking was initially developed for the separation of cations. In this report, we describe the application of pH-mediated sample stacking to anions. In this method, an electrokinetic injection is used to introduce analyte anions into the CE system and simultaneously replace the sample matrix cations with ammonia from the background electrolyte. Base is then electrokinetically injected to neutralize the sample zone and create a low conductivity region across which the analyte anions will stack. Using this method, a sensitivity enhancement of more than 66-fold was achieved without loss in separation efficiency relative to normal electrokinetic injection. Detection limits of 0.3 microM for four phenolic acids in a physiological sample were achieved using simple UV absorbance detection. The limit to the amount of sample that could be loaded using this technique was the length of the separation capillary. To further increase the amount of sample that could be loaded, a double-capillary system was developed. Using the double-capillary system the sensitivity was increased more than 300-fold and detection limits of 0.06 microM were achieved.