Ultratrace determination of selected lanthanides by luminescence enhancement

The luminescence intensity of trivalent lanthanides, especially terbium(III) and europium(III), is shown to be enhanced by coordination with the ligand 2,6-pyridinedicarboxylic acid (DPA). Further enhancement can be obtained by forming a columinescent complex aggregate with ions such as gadolinium(III), terbium(III), lanthanum(III), or yttrium(III), where the Y(III) complex shows the greatest enhancement. A surfactant, sodium dodecyl sulfate (SDS), can be added to the solution to enhance the luminescence intensity of many lanthanide-ligand complexes by segregating the complex from quenchers. In this study, SDS has little effect on the limit of detection, but it does act to extend the linear dynamic range to include higher concentrations. The combination of columinescent complex with surfactant resulted in decreased luminescence intensity coupled with an increase in the background due to the light scattered by the surfactant micelles. A mechanism for the enhancement of the lanthanide luminescent intensity by energy transfer has been described by Xu et al. This study differs in that a lanthanide ion excitation band represents the most efficient excitation path and not a ligand excitation band. The complexes examined in this study have advantages over those used in prior studies since they do not require surfactants to achieve low limits of detection (100 parts per quadrillion, ppq, for Eu(DPA)(3)(3)(-) and 60 ppq for Tb(DPA)(3)(3)(-)), and they exhibit longer linear dynamic ranges (from 4 to 6 decades) than other columinescent systems.