Enhancement of separation capability of inorganic ions by capillary electrochromatography

Abstract

An enhancement of the separation selectivity of inorganic ions by capillary electrochromatography was investigated. When a sulfo-betain-type zwitterionic surfactant as an additive in a background electrolyte (BGE) was used, the separation selectivity of the hydrophobic inorganic anions in 10 inorganic anions (chloride, bromide, iodide, iodate, nitrite, nitrate, chlorate, perchlorate, thiocyanate, and molybdate ions) and 7 cations {lithium, sodium, potassium, rubidium, cesium, magnesium, and cerium(III) ions} could be changed. The magnitude of the selectivity change was evaluated from the association constants (Kass) between the zwitterionic surfactants {3-(N,N-dimethyldodecylammonio)propane sulfonate and 3-(N,N-dimethylmyristylammonio)propane sulfonate} micelles and the inorganic ions. The largest Kass value was 800 dm3 mol-1 for the perchlorate ion among the 10 inorganic anions. The perchlorate ion could also be used an the additive in the BGE for a selectivity change of the inorganic anions. On the other hand, the Kass values for the 7 inorganic cations were small compared to those for the inorganic anions. When carboxylate ions and amines as additives in the BGE were used, the separation selectivity of copper(II) in 5 transition-metal ions {iron(II), cobalt(II), nickel(II), copper(II), and zinc(II) ions} could be signiticantly changed. The magnitude of the selectivity change was also evaluated from the stepwise coordination constants (KCuL1 and KCuL2) between [Cu(phen derivative)2]2+ (phen=1,10-phenanthroline) pre-complexed copper(II) with the phen derivatives and the carboxylate ions in the BGE and between the Cu(II)-Alizarin complexone complex anion formed by an on-line complexation and the amines in the BGE. The mechanism of the separation selectivity was considered based on the KCuL1 and KCuL2 values. Furthermore, when pyridinedicarboxylates (PDCs) as chelate reagents for the on-line complexation were used, the separation selectivity of cobalt(II) and copper(II) could be changed. The migration orders were cobalt(II)<copper(II) and copper(II)<cobalt(II) for both 2,3- and 2,5-PDC and 2,4-PDC, respectively. This result was considered based on the structures of the metal PDC complexes.