Instrumentation

Pulsed-Electron Paramagnetic Resonance Spectrometer

 

Time-resolved and trapped radical studies are performed on a home-constructed, state-of-the-art pulsed-EPR spectrometer. A guiding principle in the design of the spectrometer was flexibility, both in terms of hardware and operations software. These features allow the immediate implementation of current developments in the rapidly progressing field of pulsed-EPR spectroscopy. They also allow us to tailor the experiments as necessary for biological samples, and to develop novel microwave pulse sequences. The spectrometer operates continously over X-band (8.2-12.4 GHz) and Ku-band (12.4-18.0 GHz). Sample temperatures from <2 K to 295 K are maintained by a Janis cryostat. Home-fashioned broadband folded half wave resonators and loop gap resonators, and a modified Varian X-band cavity (TE) resonator are used for different experiments. All operations are controlled via GPIB/IEEE-488 interfaces by using LabVIEW software (National Instruments) run on a PowerMacintosh computer. Experiments that are performed include: Fourier Transform-EPR (FT-EPR), Electron spin echo-detected-EPR (ESE-EPR), and 1- and 2-dimensional n-pulse electron spin echo envelope modulation (ESEEM). Standard relaxation experiments, including saturation-recovery and inversion recovery, can also be performed. The pulsed-electron nuclear double resonance (pulsed-ENDOR) experiment is under construction, and will be on-line this summer.

A Bruker E560 X- and Q-band continuous-wave EPR spectrometer was recently installed (2004). The new EPR facility is housed in a separate room, two doors down the hall from the main Warncke Lab. The spectrometer has electron-nuclear double resonance (ENDOR) accessories for X- and Q-band. Different cryostats allow EPR and ENDOR experiments to be performed from ~4 K to room temperature. This instrument was acquired through a Shared Instrumentation Grant from the NIH (K.W., P.I.), with a generous contribution from Emory College. The user group includes members of the Physics, Chemistry and Biochemistry departments at Emory. Image shows Bruker Service Engineer, Chuck Hanson, installing and testing the instrument, while K.W. looks on.

 

A Bruker ER200D X-band continuous-wave EPR spectrometer with a Varian 12 inch electromagnet and regulator/power supply, is housed in a separate room adjacent to the Warncke Lab. The ER200D is PC-controlled. An Oxford ESR 910 continuous-flow helium cryostat and Airco nitrogen flow cryostat are used for variable temperature experiments from 4-300 K. The Bruker spectrometer and accessories are shown in the Figure.   A Varian E4 X-band EPR spectrometer is also located in the EPR facility near the Warncke Lab. This instrument is used for routine EPR measurements.

Transient Optical Spectrophotometer

Most of the projects in our laboratory involve light-triggered reactions or the preparation of light-induced metastable intermediate states. Reactant molecules undergo large, distinctive absorbance changes in the near-infrared/visible/ultraviolet (NIR/vis/UV) regions of the electromagnetic spectrum. Therefore, to both complement and extend the electron magnetic resonance studies, we are constructing a single-beam transient NIR/vis/UV spectrophotometer. This instrument is located on a 5'x10' optical table that is housed in a separate room next to the EPR spectrometer room in the Warncke Lab. The measuring wavelength spans the range ~250-1000 nm. Light excitation is provided by a xenon flash lamp pulse (2 microsecond FWHM), or, for nanosecond measurements, from the pulsed-Nd YAG laser beam routed from the EPR room. Transient absorption signals are detected with photomultiplier tubes, amplified, and digitized with a fast digital sampling oscilloscope. Data acquisition is computer controlled by using in-house LabVIEW software.

A commercial spectophotometer (Shimadzu 1601) interfaced with a PC is also located in the laboratory and used for routine spectrophotometry and enzyme assays.

Pulsed and CW Lasers

For light-excitation studies, the microwave pulse sequences are synchronized with laser pulses (10 ns FWHM) or with the chopped CW laser output. We use a pulsed Nd-YAG laser, with output at 1064, 532 and 355 nm and a CW argon ion laser. Broad-band CW Hg-Xenon and xenon flash lamps are also used. Light access to the folded halfwave resonators is afforded by sapphire windows in the cryostat and openings in the resonator mount structures. Samples in the TE cavity are irradiated through a grill. The lasers and optical components are mounted on a 4'x8' optical table that is positioned in front of the pulsed-EPR spectrometer electromagnet, as shown in the Figure.