Adsorption of buffer ion pairs can alter long-term electroosmotic flow stability

Dynamic capillary coatings have become widespread due to their efficacy in modifying the EOF in capillary electrophoretic separations and ability to limit unwanted analyte-surface interactions. However, our understanding of exactly what types of interactions are taking place at the surface of a capillary when these dynamic additives are present is limited. In this work, we have chosen a simple, small molecule additive, tetramethylammonium to examine its influence on the EOF under typical separation conditions. What we have revealed is that this simple compound does not interact with the capillary surface in a very simple manner. Our initial hypothesis of a direct ionic interaction with the silanol surface has evolved with evidence of complex ion pairing between the silanols, the tetramethylammonium, and the buffer ions. This ion pairing can result in drastic changes in the EOF over time, and that the EOF can only be restored to initial levels with harsh rinses containing sodium hydroxide.