title: 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.
authors.affiliation: Laboratory of Analytical Chemistry, Department of Chemistry, University of Helsinki, Helsinki, Finland.
authors.forename: Simo P
authors.initials: SP
authors.lastname: Porras
authors.forename: Marja-Liisa
authors.initials: ML
authors.lastname: Riekkola
authors.forename: Ernst
authors.initials: E
authors.lastname: Kenndler
mesh_terms: D000838:Anions; D001565:Benzoates / Q000032:analysis*; D002021:Buffers; D019075:Electrophoresis, Capillary / Q000379:methods* / Q000592:standards; D062385:Hydroxybenzoates / Q000032:analysis*; D007477:Ions; D008094:Lithium; D000432:Methanol; D011188:Potassium; D000644:Quaternary Ammonium Compounds; D012413:Rubidium; D012964:Sodium; D012997:Solvents; D014867:Water
chemical_list: D000838:Anions; D001565:Benzoates; D002021:Buffers; D062385:Hydroxybenzoates; D007477:Ions; D000644:Quaternary Ammonium Compounds; D012997:Solvents; D014867:Water; D008094:Lithium; D012964:Sodium; C009405:tetrabutylammonium; C017616:phenolic acid; D012413:Rubidium; D011188:Potassium; D000432:Methanol