NONAQUEOUS ANION-EXCHANGE CHROMATOGRAPHY .1. ROLE OF SOLVATION IN ANION-EXCHANGE RESIN

Nonaqueous anion-exchange chromatographic selectivity has been studied in methanol (MeOH), acetonitrile (MeCN), N,N-dimethylformamide (DMF), and their mixtures. Changing solvents results in drastic changes in a nion-exchange selectivity; in MeOH, larger and less solvated ions are more retained, while smaller and more solvated ions are more favorably retained in MeCN or DMF irrespective of the chemical structure of the anion-exchange sites. In solvents with low acceptor ability, the hydr ogen bond formation between anions and anion-exchange sites plays, a d ecisive role, and thus the affinity of small anions to the anion-excha nge sites comprised of primary ammonium ions is much higher than expec ted from electrostatic coulombic interaction. In contrast, the affinit y to the anion-exchange sites comprised of tetraalkylammonium ions is qualitatively explained by coulombic interaction in the absence of spe cific adsorption. In most cases, the ratio of k' can be explained by t he ratio of the ion-pair formation constants determined in bulk soluti on, suggesting that ion-pair formation similar to that seen in solutio n takes place in anion-exchange resins though there is a difference in the absolute values of the equilibrium constant.