Electron Paramagnetic Resonance (EPR) spectroscopy is a form of magnetic resonance (which includes techniques such as magnetic resonance imaging and nuclear magnetic resonance) that is sensitive to free radicals and other species that contain unpaired electrons. These include brown and black carbons, reactive oxygen and nitrogen species, many oxidation states of metal ions (Cu2+, Mn2+, Fe3+, V4+, Ti3+, etc.), and organic free radicals.
EMSL has a multi-frequency EPR capability, with continuous wave sources at S-band (4 GHz), X-band (9 GHz), and Q-band (35 GHz), allowing for disentanglement of the field- and non-field-dependent spectral parameters. Pulsed operation is available at X, which can probe a wide range of electron-electron and electron-nuclear couplings. These interactions are probed by experiments like electron-nuclear double resonance, electron spin echo envelope modulation, double electron-electron resonance, and hyperfine sublevel correlation.
This Bruker Elexsys spectrometer is outfitted to perform a wide range of specialty in situ measurements. The sample temperature can be varied from 1.8 K to 700 K. Custom sample holders allow for either high pressure (100 bar) or continuous gas flow through the sample. Gas is supplied by either a double high pressure syringe pump (Isco 260D) or a four-gas mixing station for low pressure flow. The spectrometer is also interfaced with a Solartron potentiostat for simultaneous collection of EPR spectra and electrochemical data. Electrochemical samples are held in either a three-electrode Wilmad flat cell (for solution state radicals) or custom-made cells (for analysis of radicals on the electrodes). These custom cells can also provide for gas or liquid flow. The sample can be illuminated in situ with a mercury lamp. It is connected to the resonator with a liquid light pipe and is equipped with bandpass filters, a timer-controlled shutter, an adjustable iris, and a chopper. Stopped flow and freeze quench samples are prepared with an Update Instruments rapid mixer.
- Supporting the Biogeochemical Transformations and Terrestrial-Atmosphere Processes Integrated Research Platforms, this resource can quantitate important, naturally occurring metal ions, such as iron and manganese, as well as organic free radicals in humics, fulvics, kerogen, and other carbon-rich soil and aerosol components. Two-dimensional pulsed experiments can give structural information about the local molecular environment of these species.
- Supporting the Rhizosphere Function Integrated Research Platform, this resource can use both ex situ and in planta experiments to observe radical species such as those created by stressors like drought and pollution. Flowing gas (labeled gases, ozone, etc.) and illumination can be provided to the sample.
- Supporting the Structural Biology Integrated Research Platform, this resource can be used to determine the structure of proteins (via site-directed spin labeling) and the structure of active sites in metalloproteins, which contain paramagnetic metals. Experiments such as double electron-electron resonance can provide distance constraints.
The instrument is referred to as “EPR Pulsed/CW Spectrometer” for the purposes of reserving experiment time.
Tips for success
- The maximum volume for most experiments is ~100 µl and corresponds to 10–50 mg of most dried material.
- The detection limit is approximately micromolar, depending on the line width of the species (i.e., 10–100 nanomolar for narrow radicals and 10–100 micromolar for very broad species such as birnessite and some iron compounds).