Factors influencing the retention of rare earth—tetraphenylporphine complexes in reversed-phase high-performance liquid chromatography

Abstract

The high-performance liquid chromatographic (HPLC) retention behaviour of the complexes of tetraphenylporphine (TPP) with twelve rare earths (REs), viz., Y(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III) and Lu(III), on an octadecyl-bonded silica gel column is described. All these metal complexes can be chromatographed without undesirable demetallation in their migration process along the column with a methanol-water mixture containing a small amount of acetyl-acetone and an amine. The elution sequence for the RE complexes depends on the amine added to the mobile phase. With trialkylamines and dialkylamines possessing branched alkyl structures, the capacity factors of the RE-TPP complexes increase monotonously in the order of increasing atomic number of the REs. With di- n-alkylamines, the capacity factor tends to decrease in the order of increasing atomic number of the REs for light to moderate lanthanides, whereas the reverse tendency occurs for the complexes of heavier lanthanides. In all instances, the capacity factor of the Y(III) complex lies between those of the Dy(III) and Ho(III) complexes. The effects of the amine are discussed in terms of the adduct complex formation of RE-TPP with the amine. Successful separation of the Nd, Gd, Tb, Dy, Ho, Er and Lu complexes in 15 min using triethylamine is demonstrated.