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Principles of extraction chromatography


Extraction chromatography combines the selectivity of liquid-liquid extraction with the rapidity of chromatographic methods. The separation of the radionuclides is based on the distribution of the cations of interest between an organic and an aqueous phase (neutral or acidic). The extractant is adsorbed on the surface of an inert support and corresponds to the organic, stationary phase.

Scheme of a resin particle



The utilized support has to be inert with respect to chemical reactions with the organic phase and the retained cations. It also has to be stable against mechanical influences and must have a large specific surface. In addition the support should be spherical in order to allow optimum packing.

The extractant, which is determining the capacity of the resin, has to have a high selectivity and needs to be chemically and physically stable.

A wide variety of extractants is used in extraction chromatography :
- Acidic extractants (e.g. HDEDP) which exchange protons against the demanded cations
- amines and ammonium salts (e.g. Aliquat 336) which exchange simple anions against anionic complexes
- organic molecules containing P=O groups (e.g. CMPO, TBP) which exchange water molecules in the hydration sphere
- crownethers and calixarenes which retain cations in function of their size

Diluents are often employed in order to avoid third-phase formation or to increase the hydrophobicity of the stationary phase.

The mobile phase is in general an acidic solution (e.g. nitric or hydrochloric acid). In order to facilitate the elution of the cations or anions of interest complexing agents (e.g. hydrofluoric acid or oxalic acid) can be added to the mobile phase.


The capacity factor k’ is proportional to the distribution ratio D used in liquid-liquid extraction. It is equivalent to the volume needed to reach the peak maximum of the analyte elution.


Vs and Vm are the volumes of the stationary and the mobile phase. In extraction chromatographic systems k’ and D are usually not measured directly, but calculated from the experimentally easily accessible weight distribution ratio Dw.

The capacity factor k’ is often given in free column values (FCV) which corresponds to the volume of the mobile phase in the interior of the column. This allows comparing methods that are using columns of different sizes.

Bibliography


T.Braun, G. Ghersini, Extraction chromatography, Journal of chromatography Library, Vol. 2  Elsevier Scientific Publishing Company 197
E. Philip Horwitz,Director, PG Research Foundation, Inc. and Senior Consulting Scientist, Eichrom Technologies,
Inc., Darien, IL 60561 USA E.P. Horwitz , Extraction Chromatography of actinides and Selected Fission Products:
 Principles and Achievement of Selectivity, website Eichrom Technologies