Ultrasensitive CE for heavy metal ions using the variations in the chemical structures formed from new octadentate fluorescent probes and cationic polymers

Abstract

Novel fluorescent probes have been developed for the ultratrace detection of heavy metal ions by capillary electrophoresis using laser-induced fluorescence detection. Based on a molecular design, the probes are composed of an octadentate chelating moiety, a macrocyclic DOTA (tetraazacyclododecanetetraacetic acid) and an acyclic DTPA (diethylenetriaminepentaacetic acid) frame, a spacer and a fluorophore (fluorescein). These were chosen on the basis of their ability to form kinetically inert and highly emissive complexes, and to prevent a quenching effect even with heavy and paramagnetic metal ions. Addition of a cationic polymer, polybrene, in the separation buffer provided high resolution and simultaneous detection of Ca²⁺, Mg²⁺, Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cd²⁺ and Pb²⁺. The direct fluorescence detection of these metal ions with high sensitivity at lower ppt levels, typically 2–7 × 10⁻¹¹ M (potentially sub-ppt), was successfully achieved. While separation of anionic compounds using a counter cation (“Ion Association (IA)” mode) is typically controlled by the ion association equilibrium constants, K_ass, it was found that differences in the mobilities, µ_ep(IAC), of the ion association complexes formed between the probe complexes and counter cations are the driving forces for separation in this new method. This suggests that each of the polybrene–probe complexes has different chemical structures among metal ions, which were able to be determined by CD spectra in this investigation. This novel separation mode was termed the “Ion Association Complex (IAC)” mode, distinct from the IA mode.