GENERAL-METHOD FOR THE DETERMINATION OF STABILITY-CONSTANTS OF LANTHANIDE ION CHELATES BY LIGAND-LIGAND COMPETITION - LASER-EXCITED EU3+ LUMINESCENCE EXCITATION SPECTROSCOPY

A highly sensitive, convenient, direct spectroscopic method for the me asurement of stability constants of Eu3+ ion complexes of multidentate ligands is described. Eu3+ ion complexation is monitored by means of (7)Fo --> (5)Do excitation spectroscopy. The method involves the compe tition against one another, of two ligands, one (L') of known and one (L) of unknown stability constant for its Eu3+ complex. With laser exc itation at a particular wavelength (in the 578-581 mm range), the exci tation intensity of EuL and/or EuL' is measured by either a time-resol ved method, if the excited state lifetimes of EuL and EuL' are suffici ently different, or in a non-time-resolved manner, if the emission int ensities (lambda(em) = 614 nm) of EuL and EuL' are significantly diffe rent at the exciting wavelength. These data lead to the determination of relative conditional stability constants (K-rel = [EuL'][L]t/[EuL][ L'](t)) of the two ligands which, with knowledge of the protonation co nstants of the ligands, can be used to calculate the thermodynamic for mation constant of the fully deprotonated Ligand, L. Using ethylenedia minetetraacetic acid (edta) as the reference ligand, formation constan ts for the 1:1 complexes of Eu3+ with N-(2-hydroxyethyl)ethylenediamin etriacetic acid (hedta), diethylenetriaminepentaacetic acid (dtpa), an d 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (do3a) were dete rmined, all in good to excellent agreement with their literature value s. The method was also applied to the Eu3+-dtpa-dien [1,4,7-tris(carbo xymetkyl)-9, 17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane] system, w here the species EuH(dtpadien)(+), Eu(dtpa-dien), and Eu(OH)(dtpa-dien )(-) exist in different pH regions. The respective formation constants are log K-EuL = 17.2 +/- 0.1, log K-EuHL = 14.2 +/- 0.2, and, for the reaction EuL + OH- = Eu(OH)L(-), log K-OH = 5.7 +/- 0.2 (pK(a) = 8.3 +/- 0.2). Advantages of the present method include high sensitivity(1- 10 pM concentrations of Eu3+ and ligand), small sample volumes (1 mL, or less), and the ability to detect directly and characterize the spec ies present in solution under particular pH conditions using excitatio n spectroscopy and lifetime measurements.