Electron Paramagnetic Resonance (EPR) is a spectroscopic technique based on the interactions of unpaired electron spins in a sample with microwaves in a magnetic field. It has widespread use in chemistry, biochemistry and physics applications.
Electrons possess a property called "spin", resulting in an angular momentum. Because the electron is charged, there is associated with the angular momentum a magnetic moment which points in the opposite direction to the angular momentum vector. In an external magnetic field, the spin precesses around the field direction at the Larmor frequency and thus a component of the magnetic moment is either parallel or anti-parallel to the field direction. If a microwave field of this frequency is applied to a spin containing sample, then the spins can change their direction relative to the magnetic field. This results in absorption of the microwave field, which may be measured.
The energy of the transition will be affected by the local environment of the spins. Thus, EPR spectra can yield information about the structure and composition of a sample.
A typical continuous wave (CW) EPR spectrometer consists of a source of microwave radiation, a cavity into which the sample is placed (to enhance the size of the microwave field), a detector to measure the microwave signal reflected from the cavity and a magnet to generate the external field. A small modulation field allows the use of phase sensitive detection.
Typically, the microwave frequency is kept constant while the magnetic field is swept. It is also possible to carry out pulsed EPR. By applying very short pulses (the order of nanoseconds) the bandwidth of the incident microwave field is increased, allowing all, or a large part of, the spectrum to be excited at once. By taking the Fourier transform of the signal that is subsequently radiated, the spectrum may be obtained.
Why High-Field EPR?
Most commercial EPR spectrometers operate at fields of around 0.3 Tesla, corresponding to microwave frequencies of about 10GHz (X band). Moving to higher fields and frequencies can offer increased sensitivity and resolution.
The equation shows the frequency and power dependence of the minimum detectable number of spins in a one Hz bandwidth, assuming that the system obeys the Bloch equations and the Curie law. The separation in field of two lines with different g-factors will scale with the field, so if the line widths remain constant, then the lines will eventually be resolved as the field is increased.
Some advantages of the high-frequency EPR are the following:
increase in spectral resolution due to the higher magnetic field;
increase in detection sensitivity for samples of limited quantity due to higher resonator filling factor;
increase in orientation selectivity in the investigation of disordered systems;
accessibility of spin systems with larger zero-field splitting due to the larger microwave quantum energy;
better separation of the ENDOR spectra of different nuclei;
improvement in the ENDOR detectability of low-frequency nuclear spins;
simplification of spectra due to the reduction of second-order effects at high fields.
At the Institute of Problems of Chemical Physics we have PS100-X and EPR 05-1 CW EPR spectrometers operating respectively at 9.7 and 140 GHz and are currently developing a 140 GHz pulsed system. Both the systems use waveguide techniques.
CW and pulsed EPR
In Nuclear Magnetic Resonance (NMR), the use of pulsed techniques has been standard for decades, but in EPR most spectrmeters systems are continuous wave (i.e. illuminate with a fixed microwave frequency and sweep the applied magnetic field). Pulse techniques, which involve illuminating the sample with sequence of short pulses then looking at what it re-radiates, offer time resolved experiments, the ability to resolve closely spaced lines and are good for measuring relaxation. To excite the whole spectrum at once, however, requires very short, very high power pulses and requires the system deadtime (the time after the last illuminating pulse before measurement can begin) to be minimised. The system dead-time often means that the signal from any broad lines has decayed before measurement starts.
Site directed spin labelling
One of the most exciting applications of EPR is that of site directed spin labelling. This technique involves attaching spin labels (usually radicals) to large molecules such as proteins and DNA. This can yield information about the immediate surroundings of the label position (is the label inside or outside the protein?) and the interaction between two or more labels can give an indication of the structure of the molecule.
High-field EPR is particularly good for this application as it has sufficient resolution to resolve the spin label spectra, making it possible to make distance measurements from 20 to 80 nm and even further in favourable circumstances. This can be combined with other distance measurement techniques such as fluorescence resonance energy transfer (FRET) to determine structure.