Europium as a label in time-resolved immunofluorometric assays

A nonisotopic immunoassay has been developed based on a sensitive detection of europium (III) in water solution using time-resolved fluorometry. The europium label is bound to the antibody with EDTA derivatives, either diazophenyl-EDTA-Eu or isothiocyanatophenyl-EDTA-Eu. After the immunometric assay has been completed the europium is preferably dissociated from the antibody at low pH and measured by time-resolved fluorescence in a micellar solution containing Triton X-100, β-diketone, and a Lewis base. The detergent solubilizes the chelating compounds in the solution and excludes water from the fluorescent ligand-europium complex. Europium concentrations as low as 5·10^?14m were measured using a 1-s counting time. The sensitivity of the immunoassay of rabbit IgG used as a model system was 25 pg/ml (6 pg/assay).