FLUORESCENCE QUENCHING OF LIQUID SCINTILLATION-COUNTERS BY METAL-IONS

The fluorescence quenching of four secondary liquid scintillation coun ters-POPOP, PBBO, alpha-NPO and BBO-by transition metal ions has been carried out in 40% methanol-water mixture at room temperature. The que nching is found to be appreciable and purely dynamic. The quenching ra te constants were determined using the Stern-Volmer equation. The mech anism of quenching by a series of transition metal ions on these scint illation counters was studied. A reasonable linear fit of the logarith m of the bimolecular quenching constant (log k(q)) with halfwave reduc tion potential (E-1/2) of the various metal ions was obtained. A simil ar 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 ele ctron transfer in nature, mediated by the formation of a nonemissive e xciplex and the direction of electron transfer is ascertained to be fr om the excited fluorophore to the metal d-orbitals, The values of Delt a 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.