Extraction chromatography versus solvent extraction: How similar are they?

Over the last decade, extraction chromatography (EXC) has emerged as a versatile and effective method for the separation and preconcentration of a number of metal ions. Frequently, EXC is described as a technique that combines the selectivity of solvent extraction (SX) with the ease of operation of chromatographic methods. Despite this, the extent to which EXC actually provides the selectivity of SX and to which solvent extraction data can be used for the quantitative prediction of the retention of metal ions on an EXC column has remained unclear. To address these questions, the extraction chromatographic and solvent extraction behavior of lanthanides using three different acidic organophosphorus extractants bis-(2-ethylhexyl)phosphoric acid (HDEHP), 2-ethylhexyl 2-ethylhexylphosphonic acid (HEH[EHP]), and bis-(2,4,4 trimethylpentyl)phosphinic acid (H[DTMPeP])) have been compared. Specifically, the rate and extent of uptake of selected lanthanides by the three extractants have been examined. In addition, the relationship between the volume distribution ratios obtained in the chromatographic and liquid-liquid extraction modes have been compared and their utility in predicting the chromatographic parameter, k', the number of free column volumes to peak maximum determined.