Fluorescence quenching of liquid scintillation counters by metal ions

The fluorescence quenching of four secondary liquid scintillation counters-POPOP, PBBO, alpha-NPO and BBO-by transition metal ions has been carried out in 40% methanol-water mixture at room temperature. The quenching is found to be appreciable and purely dynamic. The quenching rate constants were determined using the Stern-Volmer equation. The mechanism of quenching by a series of transition metal ions on these scintillation counters was studied. A reasonable linear fit of the logarithm of the bimolecular quenching constant (log k(q)) with halfwave reduction potential (E-1/2) of the various metal ions was obtained. A similar correlation between the bimolecular quenching rate constant (log k(q)) and the oxidation potential of the scintillation counters was also observed. These results indicate the mechanism of quenching to be electron transfer in nature, mediated by the formation of a nonemissive exciplex and the direction of electron transfer is ascertained to be from the excited fluorophore to the metal d-orbitals, The values of Delta G(0)(up down arrow) (the activation energy for the electron transfer process at Delta G = 0) and the free energy at which the quenching is purely diffusion limited have been approximately estimated. (C) 1998 Elsevier Science Ltd. All rights reserved.