FLOW SYSTEM AND 9.5 GHZ MICROWAVE RESONATORS FOR TIME-RESOLVED AND STEADY-STATE ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY IN COMPRESSED AND SUPERCRITICAL FLUIDS

An apparatus for the electron paramagnetic resonance (EPR) detection o f stable and transient free radicals in pressurized liquids and superc ritical fluids is described. The construction of a quartz EPR sample c ell capable of withstanding pressures up to 3000 psi (204 atm) and the components of a high pressure flow system are explained and illustrat ed. In order to accommodate the thick-walled, high pressure sample tub e, two X-band (9.5 GHz) microwave cavities were fabricated. One is a b rass, cylindrical TE011 cavity for time-resolved (direct detection) EP R experiments where field modulation was not necessary, and the other is a silver wire-wound cylindrical TE011 cavity, mounted in a quartz s upport tube. The wire-wound cavity allows for simultaneous light excit ation and field modulation for steady-state EPR. Both cavities are fit ted 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 r esonator and to allow for operation with smaller sample tubes at ambie nt pressure. Microwave coupling, flow optimization, and UV-light acces s for both cavities are described, and sample spectra in both time-res olved and steady-state modes are reported. (C) 1997 American Institute of Physics.