Effect of background electrolyte anions on markers of electroosmotic flow in capillary electrophoresis

Accurate knowledge of the electroosmotic flow mobility is crucial for the determination of effective electrophoretic mobilities in capillary electrophoresis. The addition of a neutral marker to the sample is a common method for electroosmotic flow mobility measurement. Nevertheless, the neutral marker can also exhibit electrophoretic mobility due to its interactions with background electrolyte constituents. In our previous study, we have shown the effect of different cations in background electrolytes with a fixed anion. In this work, the relative mobility of neutral compounds, either those commonly used as electroosmotic flow markers or their structurally related molecules, was measured in the presence of various anions in the background electrolyte with sodium as a fixed cation at pH 4.5. The mobility of eight compounds, formamide, N-methylformamide, N,N-dimethylformamide, acrylamide, N,N-dimethylacrylamide, N-methylacetamide, mesityl oxide, and dimethyl sulfoxide, was related to the mobility of thiourea. The studied anions were chloride, perchlorate, sulfate, and acetate. The results revealed a dramatic effect of perchlorate. The interaction with perchlorate strengthens significantly with the increasing number of non-polar moieties, such as methyl or acryl, present in the marker molecule. On the other hand, sulfate showed a very weak effect on all the studied compounds.