Capillary electrophoresis of anionic analytes in methanol: Effect of counter-ions on electrophoretic mobility

Abstract

Mobilities of 11 substituted benzoates and 3 nitrophenolates were determined in non-aqueous methanol with Li+, Na+, K+, Rb+, and tetrabutylammonium (Bu4N+) as counter-ions of the background electrolyte. The influence of the ionic concentration of the background electrolyte on the mobility of the analyte anions is more pronounced compared to aqueous solutions. The deviation from the dependence of the mobilities on the ionic strength from the Debye-Hückel-Onsager theory indicates the occurrence of ion-pair formation. For a given ion concentration (10 mmol/L), the decrease of the analyte mobility follows the counter-ion sequence Li+ < Na+ < K+ < Rb+, which is the inverse order of their Stokes radii. Bu4N+ as counter-ion has a similar effect on the analyte mobility than Li+ (which has the same Stokes radius, but a six times smaller crystal radius). Exceptions are some di- and trihydroxybenzoates. The mobilities in methanol and in water with the same counter-ion (Na+) at a given ionic concentration show very low correlation.

Recent advances in nonaqueous capillary electrophoresis

“…However, ion-pair formation seems to be more pronounced in MeOH than in ACN. Porras et al [47] have also demonstrated that the effect of BGE counterions (alkali metal ions and tetrabutylammonium ion) on the mobilities of eleven substituted benzoates and three nitrophenolates depends, as predicted by the theory, on the size of the solvated counterion.…”

Capillary zone electrophoresis in non-aqueous solutions: pH of the background electrolyte

“…As the analyte ion has typically a much lower concentration than the BGE ions, pairing does not affect the pH or ionic strength of the BGE to a considerable extent, but it clearly affects the mobility of the analyte ion. In addition, if different BGE counter ions are applied the mobility of the analyte ion may vary depending on the strength of ion-pair formation with the counter ion [54,60]. It should be mentioned that in Fig.…”

Probing the Electrostatic Shielding of DNA with Capillary Electrophoresis

“…where m is the mobility of the polyion at finite ionic strength (in units of 10 ÿ4 cm 2 V ÿ1 s ÿ1 ) and m o is the mobility at zero ionic strength (Li et al, 1999;Porras et al, 2002).…”