Effects encountered in EPR spectroscopy and imaging at small magnetic fields

Abstract

In vivo EPR studies of biological systems and other ‘wet’ systems are often performed at radiofrequencies (low static magnetic fields) in order to minimize conductive losses incurred with relatively large samples. Here, attention is drawn to several consequences which can arise from carrying out EPR spectroscopy and imaging (EMRI) at low fields. The first causes a distortion of the gradient when the applied gradient is not a small fraction of the external magnetic field. These ‘concomitant gradients’ could pose problems if high-resolution imaging experiments are attempted. Other phenomena stem from the Breit–Rabi effect which causes spectral distortions and has implications for data acquisition and image processing. A third effect is the near removal of g factor anisotropy resulting from using low magnetic fields. Thus the powder spectrum for a typical aminoxyl radical is completely different from that observed at X-band: this has consequences for both imaging and for the measurement of rotational diffusion constants. It is shown that there are disadvantages in using ¹⁵N- labelled spin probes/labels for EPR measurements at radiofrequencies.