SEPARATION OF URANIUM(VI) AND LANTHANIDES BY CAPILLARY ELECTROPHORESIS USING ON-CAPILLARY COMPLEXATION WITH ARSENATE-III

The viability of the separation of lanthanides and uranium(VI) in the form of strongly absorbing complexes with arsenate III (AIII) was stud ied with the aim to increase the sensitivity of absorbance detection i n determination of these metals by capillary electrophoresis (CE). Spe cial attention was paid to the complexation equilibria in the backgrou nd electrolyte (BGE). On-capillary complexation provided better peak s hapes for lanthanides compared to pre-column complexation. While the B GE composition had very little effect on the peak shape of the kinetic ally inert uranium(VI) complex, it played a crucial role in the peak s hapes of the kinetically labile lanthanide complexes. Addition of a se cond ligand competing with the metallochromic ligand AIII for the meta l ions was found to be critical to achieve good peak shape. The nature and concentration of the competing complexing ligand added to the BGE , the pH, and the concentration of AIII were found to exert a strong i nfluence on the separation selectivity, peak shapes and the detection sensitivity. Several carboxylic acids were compared as BGE competing l igands and citrate provided best selectivity and peak shapes. A citrat e BGE at pH 4.7 and containing 0.1 mM AIII was used for the separation of uranium(VI) (350 000 theoretical plates) and La-III (63 000 theore tical plates) while, to separate most lanthanides and uranium(VI), a s imilar BGE with a lower (0.03 mM) AIII concentration was used. Using h ydrostatic sampling (100 mm for 10 a) detection limits of 0.35 mu M (4 9 ppb) La-III and 25 mu M (60 ppb) UO2 were obtained. Using on-capilla ry complexation, sample stacking was retained for injection times of u p to at least 100 s (ca. 30-mm sample plug) without loss of peak shape s for lanthanides or recovery for La-III. When this process was used, the detection limit for La-III was reduced to about 5 ppb. Optimal pro perties of metallochromic ligands for separation and detection of meta ls by CE are discussed. (C) 1998 Elsevier Science B.V.