Investigation of the effect of ionic strength of Tris‐acetate background electrolyte on electrophoretic mobilities of mono‐, di‐, and trivalent organic anions by capillary electrophoresis

Abstract

The effect of ionic strength of the background electrolyte (BGE) composed of tris(hydroxymethyl)aminomethane (Tris) and acetic acid on the electrophoretic mobility of mono-, di- and trivalent anions of aliphatic and aromatic carboxylic and sulfonic acids was investigated by capillary zone electrophoresis (CZE). Actual ionic mobilities of the above anions were determined from their CZE separations in Tris-acetate BGEs of pH 8.1 to 8.2 in the 3 to 100 mM ionic strength interval at constant temperature (25 degrees C). It was found that the ionic strength dependence of experimentally determined actual ionic mobilities does not follow the course supposed by the classical Onsager theory. A steeper decrease of actual ionic mobilities with the increasing ionic strength of BGE and a higher estimated limiting mobility of the anions than that found in the literature could be attributed to the specific behavior of the Tris-acetate BGEs. Presumably, not only a single type of interaction of anionic analytes with BGE constituents but rather the combination of effects, such as ion association or complexation equilibria, seems to be responsible for the observed deviation of the concentration dependence of the actual ionic mobilities from the Onsager theory. Additionally, several methods for the determination of limiting ionic mobilities from CZE measured actual ionic mobilities were evaluated. It turned out that the determined limiting ionic mobilities significantly depend on the calculation procedure used.

The dependence of the electrophoretic mobility of small organic ions on ionic strength and complex formation

“…This approach can be justified in part by the fact the actual ions studied are not spherical yet the OFP theory is strictly limited to spherical particles with a centrosymmetric charge distribution. In both the works of Li et al [13] and Koval et al [14], ion association is also cited as a possible complication.…”

“…Finally, we shall also include the possibility of complex formation between the particle of interest and other species present in the solution. For organic anions [13,14], it is essential to include complex formation in order to obtain quantitative agreement between experimental and model mobilities.…”

“…Comparatively, few studies have looked carefully at the dependence of m on ionic strength and composition of the BGE and compared experimental mobilities with rigorous theoretical modeling [10,13,14]. The model used in these studies is based on the work of Onsager and Fuoss [22] and Pitts [23], and shall be referred to as the Onsager-Fuoss-Pitts, or OFP model in this study.…”

“…The OFP model was originally designed to investigate the electrical conductivity of simple salt solutions and it has been quite successful in that regard [23]. Its effectiveness in accurately accounting for the electrophoretic mobility of small peptides [10] and organic anions [13,14] is quite good if considerable liberty is taken in treating the ''effective ion radius'' as an adjustable parameter. This approach can be justified in part by the fact the actual ions studied are not spherical yet the OFP theory is strictly limited to spherical particles with a centrosymmetric charge distribution.…”

“…Significant applications include separation of short peptides [6][7][8][9][10][11][12], organic anions [13,14], proteins, and protein ''charge ladders'' [15][16][17], stereoisomers [18], and also the study of complex formation [12,[19][20][21]. There are a number of factors that influence the effectiveness of separating species in a particular system and these include the solvent, composition of BGE, temperature, pH, sample concentration, nature of the capillary (its diameter, whether it is ''coated'', etc.…”