Dynamic quenching of europium(III) and terbium(III) luminescence: a potential detection method in ion chromatography

Abstract

Indirect detection methods based on UV absorption and fluorescence measurements are well known in single-column ion chromatography’. The detector responds to the chromophoric or fluorophoric eluent ions; a negative signal is observed, as the eluting analyte displaces an equal amount of eluent at the detector. As a consequence, low detection limits are attainable only if the analytical separation is performed at low eluent concentrations. In our laboratory, we have developed an indirect detection method based on dynamic quenching of luminescence*+. Here displacement effects and therefore eluent concentrations play no role. A decrease in signal is observed for analytes able to reduce the luminescence quantum yield of a luminescing compound present in the detector cell. Previously we made use almost exclusively of the phosphorophore biacetyl, present as a solute in the eluent, and successful experiments have been reported2+. Unfortunately, the need to remove oxygen completely from the whole high-performance liquid chromatographic (HPLC) system makes the experimental set-up more complicated than for HPLC with W absorption or fluorescence detection. To reduce or even eliminate this problem we have followed two approaches. First we developed a detection technique based on an immobilized phosphorophore, covalently bound via an alkyl spacer on glass or silica gel bead@. As a second approach, reported here, we utilized rare earth metal ions as luminescent probes. Europium (III) and terbium (III) emit long-lived luminescence in fluid aqueous solutions that is hardly or not influenced by the presence of oxygen. Europium and other lanthanide ions have been applied as luminescence probes in studies on the structures of biological macromolecules7 and polymeric material@ and as NMR shift reagent@. Several non-radioisotopic immunoassays make use of Eu3 + chelates as luminescent markers lo.1 l. Because of their long luminescence lifetimes (1O-5-1O-3 s), the combination with time-resolved luminescence detection techniques results in very sensitive methods12. The use of Eu3 + and Tb3+ as detection luminophores in LC in a sensitized luminescence mode has been described by DiBella et ~1.‘~. The organic compounds are excited by UV radiation, energy transfer from the triplet state of the organic compound to an excited state of the lanthanide ion occurs and luminescence of the lanthanide is detected. Interesting results were ob-