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Description
An important aspect in capillary electrophoresis is the ability to maintain a consistent electroosmotic flow (EOF) through replicate separations; an unstable EOF can lead to inconsistencies and irreproducible results. EOF instability is often the result of the adsorption of analytes onto the silica surface during a separation. The adsorption of analytes onto the capillary surface can be reduced by altering the silica surface with a protective coating; common approaches include the use of a permanently derivatized coating or a dynamic coating. Permanent coatings can be achieved through the use of covalent chemical modifications of the silica surface however, reaction conditions are challenging to recreate and time consuming, leading to difficulties in coating/separation replication. Dynamic coatings are primarily adsorbed layers on the silica surface and can be achieved by using a range of buffer additives. Additives work by altering the net charges at the surface of the capillary; specifically the effects of the negatively charged silanol groups can be suppressed, reversed, or even enhanced. A benefit of using buffer additive coatings is that they are included in the run buffer which allows for constant regeneration of the surface coating. In addition to coatings, the implementation of a capillary rinses between runs has been shown to yield more consistent and reproducible EOF values by renewing the surface and removing any adsorbed analytes or impurities left behind after separations. Dynamic coatings have become the preferred method in modifying the EOF because of their wide range of applicability combined with their ease of use. However, the understanding of the specific types of interactions taking place at the surface of the capillary between the additives and the silanol surface is limited. Consequently, this work investigates the equilibriums established on the capillary surface and the subsequent surface dynamics that evolve over the course of multiple separations when using buffer additives. Small symmetrical tetraalkylammoniums and tetraalkylphosphoniums buffer additives; specifically tetramethylammonium, tetrabutylammonium, tetramethylphosphonium, and tetrabutylphosphonium were extensively studied. I have found that the initial hypothesize of a simple direct ionic interaction between the oniums and the silanol surface resulting in a stable long-term EOF, has not held true, as I have seen extensive changes in the EOF over time. In order to better understand the alteration in the EOF and disruption of onium adsorption on the capillary surface, a variety of solutions and buffers were tested to observe the impact on the EOF values. My observations have revealed how the EOF changes with the inclusion of different rinsing solutions and different buffering ions.
