Preconcentration of lanthanides from natural waters with lipophilic crown ether carboxylic acids for neutron activation analysis

Lipophilic crown ether carboxylic acids such as 2-(sym-dibenzo-16-crown-5-oxy)hexanoic acid and 2-(sym-dibenzo-16-crown-5-oxy)stearic acid with a cavity size comparable to the ionic radius of lanthanides are selective chelation agents for preconcentration and separation of the rare earth elements from natural waters for neutron activation analysis (NAA). Using this type of functionalized macrocyclic polyethers, quantitative extraction of lanthanides from natural waters into chloroform can be achieved at pH around 6-7. The extraction is reversible with respect to hydrogen ions, so that the complexed lanthanides can be back-extracted into a dilute nitric acid solution for NAA. A unique feature of this chelation system is that it is highly selective for lanthanides. Therefore, interfering matrix species such as sodium and bromine can be simultaneously eliminated during the extraction. Applications of this extraction method to determination of lanthanides in natural waters and in biological samples by NAA are discussed