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Influence of methanol as a buffer additive on the mobilities of organic cations in capillary electrophoresis
Author(s) -
Roy Kimberly I.,
Lucy Charles A.
Publication year - 2003
Publication title -
electrophoresis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.666
H-Index - 158
eISSN - 1522-2683
pISSN - 0173-0835
DOI - 10.1002/elps.200390047
Subject(s) - chemistry , ionic strength , solvent , dielectric , ion , methanol , ionic bonding , acetonitrile , electrophoresis , analytical chemistry (journal) , capillary electrophoresis , solvent effects , inorganic chemistry , aqueous solution , organic chemistry , materials science , chromatography , optoelectronics
The mobilities of a series of aromatic ammonium ions, ranging in charge from +1 to + 3, were investigated by capillary electrophoresis using buffers consisting of 0–75% v/v methanol. This is an extension of our previous studies involving anion mobility in methanol‐water media [1]. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. For all of the buffer compositions studied, the ionic strength effect increased with increasing cation charge, and varied as a function of solvent 1/ηε 1/2 as predicted by the electrophoretic term within the Pitts' equation. In the presence of methanol, the ionic strength effects became more dramatic. The absolute mobilities of the cations were altered by the addition of methanol to the electrophoretic media. For example, at 75% MeOH, a migration order reversal was observed between the + 2 and + 3 ammonium ions. These solvent‐induced selectivity changes are attributed to dielectric friction. As predicted by the Hubbard‐Onsager dielectric friction model, dielectric friction increased with increasing methanol content and with increasing analyte charge. Further, the changes in cation mobility correlated to the changes in solvent relaxation time (τ), ε and η. Although not predicted by the Hubbard‐Onsager theory, the + 3 ammonium ion experienced more dielectric friction than the − 3 sulfonate and − 3 carboxylate investigated previously [1]. This apparent failure of the Hubbard‐Onsager model results from its continuum nature, whereby ion‐solvent interactions are not taken into account.