Separation of transition and heavy metals using stationary phase gradients and thin layer chromatography

• Stationary phase chelation gradients can be used to separate metal ions.
• The denticity of the chelator strongly influences retention and hence separation.
• Retention factors depend on the extent to which the stationary phase is modified.
• Retention factors also depend on the degree of modification and gradient direction.
• Gradient plates modified with a tridentate ligand produced the best separations.

Stationary phase gradients for chelation thin layer chromatography (TLC) have been investigated as a tool to separate a mixture of metal ions. The gradient stationary phases were prepared using controlled rate infusion (CRI) from precursors containing mono-, bi-, and tri-dentate ligands, specifically 3-aminopropyltriethoxysilane, N-[3-(trimethoxysilyl)propyl] ethylenediamine, and N-[3-(trimethoxysilyl)propyl] diethylenetriamine. The presence and the extent of gradient formation were confirmed using N1s X-ray photoelectron spectroscopy (XPS). XPS results showed that the degree of modification was dependent on the aminosilane precursor, its concentration, and the rate of infusion. The separation of four transition and heavy metals (Co2+, Pb2+, Cu2+, and Fe3+) on gradient and uniformly modified plates was compared using a mobile phase containing a stronger chelating agent, ethylenediaminetetraacetic acid (EDTA). The retention of the metal ions was manipulated by varying the surface concentration of the chelating ligands. The order of retention on unmodified plates and on plates modified with a monodentate ligand was Fe3+ > Cu2+ ∼ Pb2+ ∼ Co2+, while the order of retention on plates modified with bi- and tri-dentate ligands was Fe3+ > Cu2+ > Pb2+ ∼ Co2+. Fe3+ and Cu2+ were much more sensitive to the concentration of chelating ligand on the surface (displaying lower Rf values with increasing ligand concentration) than Pb2+ and Co2+. Complete separation was achieved using a high concentration of the tridentate ligand coupled with a longer time for modification, yielding a retention order of Fe3+ > Cu2+ > Co2+ > Pb2+.