Stacking Neutral Analytes in Capillary Electrokinetic Chromatography with High-Salt Sample Matrixes

We recently described a method to stack neutral analytes in electrokinetic chromatography dependent upon using high-salt sample matrixes (Palmer et al. Anal. Chem. 1999, 71, 1679−1687) The use of a high-mobility co-ion in the sample matrix at a higher concentration than the like-charged electrokinetic vector (e.g., charged micelle) in the separation buffer as we previously suggested induces a single-boundary, discontinuous buffer system which subsequently stacks neutral analytes. Under these conditions, it is possible to electrokinetically inject neutral analytes directly from a high-salt sample matrix via electroosmotic flow, with stacking initiated simultaneously with injection. The correspondence by Quirino et al. ( Anal. Chem., previous paper in this issue) suggests the effects of the high-salt sample matrix on stacking of micelles is obviated by a destacking of the micelle zone as it exits the sample zone. They reevaluate the high-salt stacking mode in terms of the γ function. Based on the present data, it is our interpretation that the stacking efficiency is not accurately represented by the γ function and that neither micelle zone nor neutral analyte destacking occurs under conditions we previously described. Low stacking efficiency when low-conductivity sample matrixes are used indicates that the high-salt stacking effect cannot be explained simply as a function of the k value of an analyte/micelle system.