Fluorescence determination of closed-shell rare earth metal ions by reversed-phase HPLC with precolumn derivatization using 8-amino-2-[(2-amino-5-methylphenoxy)methyl]-6-methoxyquinoline-N,N,N′, N′-tetraacetate

A sensitive and selective determination of closed-shell rare earth metal ions (La³⁺, Lu³⁺, and Y³⁺) has been demonstrated using a pre-column chelating reagent {8-amino-2-[(2-amino-5-methylphenoxy)methyl]-6-methoxyquinoline-N,N,N′, N′-tetraacetatic acid (Quin2)}, in reversed-phase HPLC. Although the resolution is still unsatisfactory in this HPLC system, rare- earth metal chelates with this octadentate aromatic polyaminocarboxylate can be detected. This result suggests that these chelates are unexpectedly inert for the dissociation process during elution. The order of elution is quite different from that of the conventional ion-exchange mode separation. Middle-lanthanide ion-Quin2 chelates were most rapidly eluted, followed by the heavy ones; the light ones were the most strongly retained. Since ligand-centered fluorescence was observed for only La³⁺, Lu³⁺, Y³⁺, and Gd³⁺-Quin2 chelates, and other rare earth metal ion chelates were sufficiently quenched through a paramagnetic deactivation process, these four metal ions were selectively detected by fluorimetric detection (λ_ex= 335 nm, λ_em= 500 nm). The detection limits of La³⁺, Lu³⁺, and Y³⁺ were 2.2, 4.0, and 1.9 ppb, respectively, on a 3σ basis. No interference from each 10-fold excess of other rare-earth metal ions was observed. The success of this HPLC system for rare- earth ions is derived from the combined properties of the Quin2 chelates in terms of inertness, Chromatographic retention and fluorescence detection.