Flow system and 9.5 GHz microwave resonators for time-resolved and steady-state electron paramagnetic resonance spectroscopy in compressed and supercritical fluids

Abstract

An apparatus for the electron paramagnetic resonance (EPR) detection of stable and transient free radicals in pressurized liquids and supercritical fluids is described. The construction of a quartz EPR sample cell capable of withstanding pressures up to 3000 psi (204 atm) and the components of a high pressure flow system are explained and illustrated. In order to accommodate the thick-walled, high pressure sample tube, two X-band (9.5 GHz) microwave cavities were fabricated. One is a brass, cylindrical TE011 cavity for time-resolved (direct detection) EPR experiments where field modulation was not necessary, and the other is a silver wire-wound cylindrical TE011 cavity, mounted in a quartz support tube. The wire-wound cavity allows for simultaneous light excitation and field modulation for steady-state EPR. Both cavities are fitted with tunable end plates to facilitate tuning at X band in the face of large frequency shifts when the sample tube is introduced to the resonator and to allow for operation with smaller sample tubes at ambient pressure. Microwave coupling, flow optimization, and UV-light access for both cavities are described, and sample spectra in both time-resolved and steady-state modes are reported.