MACROCYCLE-BASED COLUMN FOR THE SEPARATION OF INORGANIC CATIONS BY ION CHROMATOGRAPHY

A tetradecyl-18-crown-6 (TD18C6) column previously used exclusively for anion separations, has been successfully applied to the separation of mono- and divalent cations. The separation is dependent on the selectivity of the macrocycle for inorganic cations. Because of the unusual selectivity exhibited by TD18C6, this macrocycle based stationary phase is suitable for the simultaneous separation of three alkaline earth metal cations, five alkali metal cations and the ammonium ion. Due to the similar selectivity of TD18C6 for Mg2+ and Ca2+, these ions coelute. The affinity of this column for the hydronium ion is great enough that the cations of interest can be eluted by mildly acidic eluents which are amenable to chemical suppression. The innovations of the macrocycle-based cation chromatographic separation system described herein are fourfold. First, due to the high capacity of the columns, an acidic eluent is employed resulting in a different selectivity than previously seen. The selectivity is unique among macrocycle-based columns because there is no variability in retention times resulting from different counter-anions associated with the analytes. Second, the stationary phase is composed of macrocycle adsorbed onto a non-polar polymer resin, rather than to a silica-based column. This allows for the use of basic eluents for the separation of anions without fear of destroying the polymeric substrate. Third, tetradecyl-substituted 18-crown-6 has not been used in other reported separation schemes of this type. Fourth, this system allows for a greater sensitivity for Cs+ because it elutes early in the analysis without coelution. Gradient separations employing organic modifiers, temperature, and pH provided improvements in separation over isocratic analyses.