EFFECTS ENCOUNTERED IN EPR SPECTROSCOPY AND IMAGING AT SMALL MAGNETIC-FIELDS

In vivo EPR studies of biological systems and other 'wet' systems are often performed at radiofrequencies (low static magnetic fields) in or der to minimize conductive losses incurred with relatively large sampl es. Here, attention is drawn to several consequences which can arise f rom carrying out EPR spectroscopy and imaging (EMRI) at low fields. Th e first causes a distortion of the gradient when the applied gradient is not a small fraction of the external magnetic field. These 'concomi tant gradients' could pose problems if high-resolution imaging experim ents are attempted. Other phenomena stem from the Breit-Rabi effect wh ich causes spectral distortions and has implications for data acquisit ion and image processing. A third effect is the near removal of g fact or anisotropy resulting from using low magnetic fields. Thus the powde r spectrum for a typical aminoxyl radical is completely different from that observed at X-band: this has consequences for both imaging and f or the measurement of rotational diffusion constants. It is shown that there are disadvantages in using N-15- labelled spin probes/labels fo r EPR measurements at radiofrequencies.