The determination of trace levels of thorium(IV) and uranyl by reversed-phase chromatography with on-line preconcentration and ligand-exchange

A method for the preconcentration of thorium(IV) and uranyl ions has been developed using pre-column complexation and loading of the complexes onto a short C18 concentrator column. It was found that the most important factor contributing to the preconcentration of thorium(IV) and uranyl on a hydrophobic stationary phase was the nature of the ligand added to the sample. Of the ligands investigated, mandelic acid gave the highest recoveries and maximum retention (highest breakthrough volume) of the analytes was achieved using sample solutions containing 42 mM mandelate. Following the preconcentration step, analysis of the sample was performed using a ì-Bondapak C18 column with an eluent comprising 200 mM á-hydroxyisobutyric acid and 10% methanol, adjusted to pH 4.0. The higher formation constants of the solutes with á-hydroxyisobutyric acid (HIBA) resulted in a quantitative ligand-exchange reaction so that the solutes were separated as their HIBA complexes. A linear relationship between the peak area and both the sample concentration and sample loading volume (up to 50 ml) for thorium(IV) and uranyl was obtained with detection limits in the sub-ìg/l range. Most common anions did not affect the preconcentration when present at concentrations of up to 1.0 M, with the exception of sulfate and acetate. The lanthanides and some transition metals which also formed complexes with mandelic acid were also trapped on the concentrator column and the resulting peaks partially overlapped the thorium(IV) peak in the final analysis. This problem was overcome by using a longer concentrator column. The technique has been applied successfully to the determination of ìg/l levels of thorium(IV) and uranyl spiked in sea water.