title: An advanced EPR stopped-flow apparatus based on a dielectric ring resonator.
abstract: A novel EPR stopped-flow accessory is described which allows time-dependent cw-EPR measurements of rate constants of reactions involving paramagnetic species after rapid mixing of two liquid reagents. The EPR stopped-flow design represents a state-of-the-art, computer controlled fluid driving system, a miniresonant EPR structure with an integrated small ball mixer, and a stopping valve. The X-band EPR detection system is an improved version of that reported by Sienkiewicz et al. [Rev. Sci. Instr. 65 (1994) 68], and utilizes a resonator with two stacked ceramic dielectric rings separated by a variable spacer. The resonator with the mode TE(H)011 is tailored particularly for conditions of fast flowing and rapidly stopped aqueous solutions, and for a high time resolution. The short distance between the ball mixer and the small EPR active volume (1.8 microl) yields a measured dead time of 330 micros. A compact assembly of all parts results in minimization of disturbing microphonics. The computer controlled driving system from BioLogic with two independent stepping motors was optimized for EPR stopped-flow with a hard-stop valve. Performance tests on the EPR spectrometer ESP 300E from BRUKER using redox reactions of nitroxide radicals revealed the EPR stopped-flow accessory as an advanced, versatile, and reliable instrument with high reproducibility.
authors.affiliation: Max Planck Institute for Bioinorganic Chemistry, D-45413 Mülheim/Ruhr, Germany. lassmann@mpi-muelheim.mpg.de
authors.forename: Günter
authors.initials: G
authors.lastname: Lassmann
authors.forename: Peter Paul
authors.initials: PP
authors.lastname: Schmidt
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lubitz
mesh_terms: D004578:Electron Spin Resonance Spectroscopy / Q000295:instrumentation*; D004867:Equipment Design; D012212:Rheology / Q000295:instrumentation*